I agree, two female Convicts would be good. But only if the Mystery Snails are no longer in the tank. For pleco's, Bristlenose or Clown would be fine. But as with the snails, have to remember to actually feed the pleco's (algae wafers/zucchini/driftwood). Is he doing weekly water changes? Pleco's, even small ones poop a lot.
Are the Mollies, or any fish with ich still in the tank? Or is the tank empty?
If it's empty, you can go ahead and clean it all out, and if you have extra media in your filter, or an extra filter, put it on his tank or extra media in his filter. If there is still fish in the tank, then I'd say treat for ich and up the temp for 10 days, to get rid of the ich then do the filter switch.
You should try (if it's possible) to teach his gf the basics. I mean, it's not THAT hard to keep fish alive and healthy, haha.
Maybe she should join on here? or give her informative websites to check out.