To Cycle his tank, why not put his filter on one of your tanks for a month or so? That way, his tank will be cycled and he can put fish in his tank right before switching his filter back onto his tank.
Make sure he is actually feeding the Mystery Snails. Zucchini, Romaine Lettuce, and Algae Wafers are all good. Convicts + Snails is a bad mix. Make sure he gets something that will not pick at/kill the snails.
There is not many cichlids that are good for a 20 gallon, maybe since he is new to fish keeping, he should keep it simple with Mollies, Platies, or a school of Tetras.