Well, as I found out just the other day, imprinting does not mean he has fallen in প্রণয় with her, it means he will be whatever She' needs. So if She' falls in প্রণয় with someone else, he'll still be there to protect her.
He'd be jealous and really hurt. He wouldn't physically attack the other guy, unless that guy was a jerk and hurt Nessie, because otherwise doing so would be hurting her. He'd try to win her over and prove that he's better for her though. In the long term, the devotion of the imprint bond would win.