I actually think the other way around. Refusing to help out a friend because it would inconvenience you? Scummy. Refusing to help out a friend because it’s against your moral principles? Not scummy.
It’d depend on the specific friend and the specific principle involved, though. I have friends for whom I’d do this one in a heartbeat, but I wouldn’t cover for them having affairs. This is, IMO, in a completely different league. If you’re in a position to know that this is a sound, hardworking, reliable, intelligent guy, then saying so to a prospective employer - even if you lie about how you know - is not the same as saying, ‘Oh, hi, Joe! Yes, of course Jane and I spent the weekend hanging out and painting our toenails. She’s just, um, having a shower, I’ll get her to call you back…’
But yeah, I’d be pissed off with him for not asking me first.