Well, then it will be up to you to maintain your sense of self and not slink away when she's there. You will have to step up your efforts to have a polite relationship with her, and if she is dismissive of you do not let it go by. You will have to stand your ground as someone who has a place in his life. If there are things you won't stand for, don't let them get away with shit.
Wait - who will sleep on the couch when you are there - you or her? I thought she was moving in, but into a spare room. She doesn't have to move in and into his bedroom, does she? Isn't there a space in the house she can designate as her room, separate from his?
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan