I didn't find Mnemba rustic either - just (very!) pared back luxury. Found the diving way superior to anything in Maldives and i've dived a lot in the maldives, deep sea fishing excellent, had fun body boarding on one day, great food. I don't remember lying around on the beach too much (although i do actually enjoy that!) so wouldn't imagine your friend would be bored.
Also enjoyed Stone Town - stayed in Serena Inn and found it fine - nothing special in itself but a more than adequate base for what is a very interesting town.