I compare it to a father's relationship with his daughter. Daughter is dating the absolute wrong person right now (drug use, unemployed, etc). He's gone deeper and deeper into bad elements over the years. You've hoped, wished and supported the boyfriend turns his life around for years while watching this all torment your daughter.
She's been loyal, but you're daughter's now at the point where one more f-up by the boyfriend and she's kicking him to the curb. That breakup will be painful, but better for the long-term.
Let the rest of this season be that one relapse.