The 2 main operators in Lahaina are Lahaina Divers and Extended Horizons. Do a search on the internet and you can read about what each offer. Biggest differences are: Lahaina Divers has scheduled charters to Molokini, Lanai, W. Maui, and Molokai, whereas Extended Horizons does W. Maui and Lanai; Extended Horizons takes 12 divers whereas Lahaina Divers takes up to 24; Lahaina Divers big 46' Newton dive boats are super comfortable on an often rough crossing, Extended Horizons has a smaller aluminum boat that can be a rough ride; Lahaina Divers is very geared toward the less experienced divers, Extended Horizons has more of a mix; Lahaina Divers will put 8 divers with a guide, Extended Horizons limits it to 6 per group.....and I'm sure there are more, but those are the ones I can think of off the top of my head.
IMO it is worth getting up early and making the 45 minute drive from the Lahaina area to Kihei to catch a charter for S. Maui and Molokini sites. Most recommended boat operators around here are Ed Robinson's, B&B, and Mike Severns (though I also recommend Makena Coast Charters).
However, if you want to save a bit of money and avoid the super early check-in (5:45am-6:15am depending on the operator), you can always do guided shore diving in S. Maui with Shaka Doug. He doesn't get rolling until 9am and you simply meet him at the site he tells you and he takes care of the rest.
In late August this year, the water temperature was only 77-78 degrees and in early July it was 75-76. If you use those temperatures as you're guide, you'll be fine. For someone 90 pounds, I would stick with the 7/5 even in the summer. She can always let a bit of water in her suit if she's warm underwater.
When will you be in the islands?