Ack I have been there many times , even with rehabed mandys

they produce toxins when stressed . In shipping they pretty much ingest a lethal dose but it takes time to effect them usually for me about 3 months into being happy and healthy eating pods and getting fat they just slip into "cardiac " mode when they stop swimming and look like they cant breathe out of the 4 or so I have tried to rescue from the stores over time I had 1 live longer than 4-5 months. I hope your buddy makes the turn for the better , I have tried alot of things QT ect .... but nothing ever worked I was gonna try to super aerate the water it was in once but never got the chance to. best of luck