This is very true, but. She knows the mortgage deal ends in July. By placing a flag on it, he can no longer negotiate for the next fixed-term deal and they may not wish to continue and offer him a variable rate deal. In this situation he would be forced to sell.
I am hoping that proving he has paid the mortgage on his own with no financial input from her for the past 18 months will mean he can continue paying the mortgage on whatever deal he can get. Issue is that she is still co-owner and maybe has to sign any new deal, which she won't. So where does that leave him?