Because it's easier for your MIL to live in denial than face the truth. This is what battered women do too. She's elderly and there is nothing you can say or do to change her. She must be getting something out of it or she wouldn't keep doing it. Harsh, I know but she seems to have pulled some abusers into her life like the boyfriend. Something inside of her tells her she can't do better. If I were you, I'd have a very long and hard talk with that boyfriend and tell him he better not ever bring anymore discomfort into MIL's life or he'll have to deal with you and the rest of the family. That's ridiculous.

Like my daughter's sitution, she grew tired of trying to talk the truth to her inlaws and now, she tells them she doesn't want to hear about their daughter anymore because all they've done is make it easier for her to do drugs. It's up to your MIL. There isn't anything you can do about her daughter. It's a choice.