First of all, I think jake might have known it was going to happen before hand! You have to remember that at the time this book was written, everyone in the group was trying to be friendly with Ax, to gain his trust, and to learn his secrets. Part of me wonders if Jake himself didn't, after hearing Cassie talk about how she thought Ax was lonely, order Cassie to bring Ax to meet with her family to try and get him to open up. Even if Jake didn't know about it beforehand, he trusts Cassies judgement. Jake has repeated how much he trusts cassie's opinion with people, and her ideas when it comes to human beings, in nearly every book he was the narrator in.
I don't really think he'd feel creeped out or bothered at all, if anything, I think he would feel MORE love for cassie because of her kind heart.