Yikes, that is a lot of cats with sensitive stomachs. But, the best thing I know that I would try first would be to keep the cats off of any 'fishy' foods. Seems like my cats that are that way would vomit sometimes from too much fishy canned food, especially tuna.
I would keep with chicken or turkey as the main meat, if they were my cats. But, I am sure others will be on here to give you some ideas.
A cat with crystals, definitely should not have too much fishy food due to phosphorous levels, etc.
My Ketchie has Interstitial Cystitis, and I have had her mainly eating Fancy Feast turkey, chicken, and also it might be a good idea to not give foods with too many additives such as wheat gluten, etc. Also, adding water to the wet food might help with the cats too.
Your friend sure has her hands full! Good luck!
Jean and Charcoal (GA)