The Back Wall is not for snorkelers or future divers - it's often a live drop, vertical, deep, there's enough current that they'll pick you up farther along the wall so a snorkeler might actually find it unsafe.
Options where you can do both - there's others but these are ones I know of.
Trilogy Sailing Charters does both - actually the snorkeling is probably better than the diving sometimes based on where they go. They also offer SNUBA which could be an intro to diving for her. Sort of higher priced for Scuba compared to other options.
Pride of Maui is a big snorkeling powered cat out of Ma'alea. They also offer intro to scuba and if you're certified will probvide tanks/weights etc. and turn you loose. They moor inside the crater so it's usually pretty calm - also they drop steps into the water. Lunch is included in the price but it's higher priced than a dive trip with a dive operation. Some of their trips go up the coast to Kapalua also.
Maui Diamond II is also based in Ma'alea - it's a smaller boat that's booked either direct or thru Maui Dreams Dive Co. in Kihei - they offer both options on the same trip often.
There's going to be 10x the snorkelers inside Molokini Crater vs. divers on any given day. Mention it if you book a dive trip - they may have site options where you can do both. Sometimes they hang off the tips of the crater so that's more for diving but if they moor inside, you go one direction and she goes the other. I don't know if any of the dive boats offer snorkel guides but someone stays on the boat so they could watch out for your wife.
Some of the Lanai options might be possible also. Trilogy snorkels there besides sailing over to Molokini. They used to (maybe still do) manage the snorkel/dive shop at the Four Seasons - the snorkel in Hulopoe bay there is one of the best in Hawaii - might see Spinner Dolphins playing there. I don't know where you'd get tanks on Lanai if Trilogy no longer has the concession though.
We dove Sharkfin some years ago with Lahaina Divers and they moored in the bay and some of the group went towards Lanai where it's pretty shallow to find the octo's. Instabuddy and I went around the outside which was an e-ticket ride, lots of current, a big drop to past rec depths and thousands of fish holding in the current - white tips patrolling around the perimeter.
Lastly some of the Maui shore dives are good snorkels also. At Black Rock - once you get to the back, it's not uncommon for snorkeler's to swim down for a look and she could stay above you for the entire dive - it's not very deep. They do both off Kapalua Beach also - get tanks at the Maui Dive Shop in Kahana Gateway - it's on the way.
At Wailea Beach the entire south side is a decent snorkel to maybe 30' it only gets to be about 60-70 a good swim out. I've done both there - there's kind of a shelf along the left side so it's pretty shallow till you get to the point where it's maybe 40'? We dove there, the Grand Wailea next door has a dive shop.
Just for reference, I recognize 8/10 of these as dive sites and have dove several.
http://www.prideofmaui.com/blog/snorkeling-maui/top-5-snorkeling-spots-in-maui.html
Four years later though I still haven't made much progress sadly.
