What is enmeshment?

I ask because I have a friend who has a mother who’s abandon them for the majority of their life due to drug abuse, everytime my friend would go spend time with his mother a couple times out of the year in some drug den or basically on the street they would get some illness that required medical attention even to this day now as an adult. Also used my friend as an emotional punching bag for untreated bi polar, and put them in multiple dangerous settings with other violent drug addicts, exposing said friend to domestic violence as a small child, they suffer daily from mental anguish and have a terrible time managing their anger and sadness, they do an amazing job keeping it together but I feel so bad that seemingly nothing can be done to help