The 30 days thing is interesting. I wonder if it is cumulative game time if that is indeed a thing? He bought the game a year ago, joined the FC, and then didn't play at all. Just restarted the other day and I've been walking him through things.

As for all the other suggestions, those were all tried (see first message and list of things tried). It's been completely perplexing why he can't stable. Everything I know about what impacts stabling (FC restrictions, Companion summoned, or failed to do quest) have all been verified.