I think his choice of not making his mother's hoarding his problem isn't actually a bad one, if he isn't financially or emotionally enabling it. It's more or less impossible to help someone who doesn't think they have a problem or want to change.

If you live across the country, can her life really interfere that much with yours? Do you think your boyfriend will start picking up the financial slack once his father stops supporting his mother?

I think I would be reluctant to make this my problem unless the way your fiance handles it interferes with your life.