So many times I hear about individuals who were hurt by a lover yet they don't want to let go of the relationship. They hold onto the hope that things will be good in the future - even if they haven't been good in the past - if they can only hold on.
As an outsider, it just doesn't make sense. Those who are caught in the unhealthy situation are often attractive, intelligent and kind. They are willing to give of themselves for the benefit of the other person but don't seem to be able to protect themselves from further hurts. And so they hang on and hang on, forgiving over and over again but things do not improve.
There are over seven billion in the world now and yet some people think that their whole being should be focused on the relationship that they have had with just this one individual. They overlook the cheating, abuse and neglect and even, sometimes, blame themselves for the inappropriate treatment that they have received. They think that if they were just more lovable, they would be loved more.
When they are left, after a breakup, they grieve deeply. When the ex makes contact again, however, they find themselves caught in a spiral of obsession that causes them to neglect other parts of their lives and focus on the dream of being partnered with this person again. They will accept the term "friendship" when it is offered but secretly cling to the idea that it will eventually turn to romance. Many truly believe that this is the only person who will love them and to be able to meet their needs - even when they don't.
In fact, when the person gets really honest about things, s/he will realize that out of all the things that they need, the ex was able to fill only one or two of them - and at a price! It might be the sex, or the attention that can fill what might otherwise be lonely hours but neither of these can make up for the long-term disappointments and devastation that accompany negative behaviours and neglect.
It's a sad situation to watch someone who you care about get caught in this type of situation. But you can't make decisions for them and usually can't talk them out of the dilemma. In fact, there are times that they might even pull away from you if they think that you are not supporting their hopes for the relationship.
You can't change this person any more than they can change the person who has hurt them.
Rather than jeopardizing your relationship with this person, why not suggest that they meet with a psychologist who is trained and can help them work through the issues? That way, they will benefit from the expertise of a professional and still have you as a good support.