Not a good situation
Doesn't seem like a good situation for anyone to me. It's hard to get a complete picture of all the cast from your writing but it "seems" that his SO has has no interest in sharing anything (may or not be accurate?) and no interest in really investigating the pros & cons at all. Again, from your writing, is seems monogamy is her choice/belief system ?
If indeed this is the case I think you'll have to accept the fact that given it's current direction, eventually he is going to be forced to make a choice. Or it may be made for him. Somebody's (everybody?) going to probably get hurt.
So I guess I'd be prepared for the worst.