i've been surfing most of my life and have taught many people to surf on the big island. given your parameters, i think this is the best course of action (which should result in the most fun for everyone).
it sounds like the first hurdle is having the hosts - you - understand that this is a perfectly reasonable thing to do, and probably a dream for her. for anyone that doesn't live in hawaii and dreams about surfing - it is an absolute dream come true to come here and get to actually do it. success will not be measured by how much she stands up or how big a wave she rides - it is measured on her getting to actualize her dream with reality. getting to actually go to the ocean and be a part of the magic that happens when you go to a surfing spot will be much a part of the success.
Hawaii, and the big island, are first and foremost very dangerous places to go in the ocean. You're not going to have a lot of success driving around looking for different surf spots for different swells. its just too big and confusing, even for a seasoned veteran. fortunately for you, HONOLII is a perfectly good place to learn to surf - probably the best place you'll find on the east side of the big island. trying to go to kona to surf is challenging and usually only results in frustration. pohoiki is a good surf spot, but not at all for beginners. if she can get to honolii for 3 or 4 sessions (or if you're really good hosts then once a day) then that will be plenty of time for her to get a real 'experience'.
so that settles the where and how often of it - the next question is when. for a beginner, the best time is when there is the most amount of water on the rocks (cause it's all rocks at honolii - but generally round smooth rocks). HIGH TIDE will always be your best bet. But high tide only lasts for a minute, then it starts going low again for the next 6 hours. The absolute best time for a surf session will be 2-3 hours before high tide. that way the influx of water will always be coming in. when the tide starts to go low, the waves generally (not always) are not as good. you're not so concerned about wave quality, but 3 hours before high tide, leading up to high tide is your prime time. another beautiful time to be in the water when conditions are best will be within 3 hours of sunrise and sunset. the winds are generally lighter and the surface conditions of the water much improved which makes everything easier. plus the sky does all these magical things at those times which will do nothing but enhance the whole experience. you can look at a tide chart and pretty much make your surfing schedule from that weeks in advance.
the best place for beginners at Honolii is on the inside near the rivermouth. you won't want to go to the outside where most of the other surfers are. it's too big and she'll just get in the way. its shallower and the waves are smaller by the river, so stay in the river. the parents, or whoever, can also wade out in the river too and mingle with her while she is surfing. that is also a lot of fun, and feels absolutely great (a little cold at first) and is a beautiful viewpoint. if you don't have a board and are wading don't go past waist deep as there is a little undercurrent from the river going out to see. (by the way if it just rained really hard the night before then the river will be running pretty good and the water will be brown and dirty. this is not a good time to go surfing, as the waves on the inside won't be as good, and more sharks will be in the water. go look at waterfalls instead which will be doing there thing at that time)
so now that you know where and when to go - how do you actually do it? honolii is a good place to learn because there are lifeguards on duty - but they are a last resort. you're going to want someone in the water with you telling exactly where to sit, where it is safe to be and not safe to be, and what waves should try to be caught. these factors change on a day to day basis and swell and wind conditions change. for at least the first 2 sessions, i would hire a surf instructor - or if there is someone you know and really trust you can use them. a surf instructor will make your experience in the water very satisfying. it is worth the money. you could probably find somebody for $50 per session. they may even provide a board for her to ride. after 2 or 3 sessions with a surf instructor she may feel comfortable enough to go on her own. try ask at orchidland surf shop or look on craigslist. you're going to be putting her life in this persons hands, so choose wisely (and don't be cheap - how often does she get to come to hawaii)
you may be able to negotiate a deal with the surf instructor for the board that she uses to be rented for the entire week. best and safest bet will be some type of longer board. at least 7 feet up to about 9 feet depending on how big she is. they also make foam boards which are much safer and every bit as surfable for a beginner as a fiberglass/standard board. it's best to stick with the same type of board all week - switching boards will just add to the confusion of it all. you'll probably need some surf racks too - you can rent those soft racks, and just keep them on the car all week. they are kind of a pain to put on and off the car. plus it just lends to the experience of surfing if you've always got the racks on your car.
make sure you eat a good meal a couple hours before being in the ocean - a lot of energy will be exerted, and you don't want to have to stop to go and get some food. it's time consuming enough just going down to the surf spot to surf for two hours. don't muddle that up with "i'm hungry" or "we need to go and get some food". take care all of that before hand, and bring some snacks with you. there is a beautiful grassy area at Honolii and restrooms and showers, so it should be no problem for other people in the party to stay occupied and comfortable while the surfing is happening.
Also be sure when you get there to just watch the surfers for at least 15 minutes, you'll get a good idea of what kind of experience you may have based on what kind of experience everyone else is having. is it just a bunch of older surfers going down to the water with stoic looks on their faces? it may be because the waves are huge or dangerous and they know they are in for a beating. are families down there and other young children in the water? that may be the perfect time. how good a time everyone else is having will somewhat translate into how good of a time you are going to have.
gypsy69 also gives some good advice above. best of luck
