What should you do if a family member is dating someone who will cause the downfall and ruination of them?

Asked by msh (4270) September 9th, 2015 from iPhone

Every family member, including their grown offspring, their best friends, etc. have told this particular family member to run- not walk away from this relationship.
This ‘beloved’ person is morally corrupt, really nasty and a mean, controlling individual. (I could go on.) But this family member is getting in so deep with ‘schnookums’ that they are turning a blind eye to some of the illegal activity going on all around this family member in the house where they live.
This awful person’s underage child, the child’s boyfriend and all the extra illegal and unethical activities the kids are also into, add more fodder for trouble.
This family member does NOT participate in these goings-on, but does not believe that just by being present- they will be charged also if a problem gets really serious. This family member was not raised to act nor be around such people. They have never, ever, acted this way before.
Any suggestions? Buy them a bigger shovel to make the hole they’re digging themself go faster? Ship them off for a stint in the French Foreign Legion ” to forget”. Or just watch on the sidelines and bite one’s tongue? This is so upsetting and frightening to all of us. Jellies?