By rebuilding it. The more was done, more it'll have to be proven, with words and actions, that they can trust again. Forgiveness is automatic, leaving the door open to reconciliation and restoration of relationship. But,trust is regained, as they are not obligated to put themselves at risk again.
Also, trying to force things won't work if they need their space and may only make things worse. I tried, and it only made things worse. I think it should be made known that there is acknowledgement of the mistake, regret over it and a willingness to do what is needed to correct it and restore the relationship. But they have the reins.