If you have found that both of them need help then your option is to try to engage them in their own process. Offer to find a therapist or even a home health worker in the States so that they can see there is help available. If you are too remote to do so you have to empower them to find this help. Make regular calls to encourage them to ask for help. Enlist your dad in admitting to his part in enabling such a relationship possibly through the AlAnon process. He may find that his being a part of this is actually supporting negative behavior. He has to want to leave and carry this out. But he may have to be educated on a codependent relationship and that is often by asking for help for his part. When he learns he doesn't have to support a toxic relationship he will find a way to leave.
Be back tomorrow