It is possible that your roommate has forgiven her boyfriend only to a point, but not completely. It seems that she still has residual anger that is bothering her. Also, even if she has forgiven him, she now may be struggling with the issue of reconciliation, or whether or not to continue the relationship. If she is talking about this as a call for help from you, then you might ask what her level of trust is with the boyfriend. See if she is struggling with the issue of reconciliation. It could be that she a) has forgiven, but not deeply yet, in which case she needs more time in the forgiveness process, or b) she is struggling with the issue of whether to reconcile or not.
Learn more at Forgiveness for Couples.