Baby daddy on heroin

My baby daddy / boyfriend is a “recovering addict” or so I thought. I went on his phone because I could tell he’s been behaving differently and hiding something and no I’m not ashamed to say that I did it because I won’t have him around our daughter if he’s on drugs. Doing so I discovered he’s using pills again and I read another message from a guy telling him to come over and they could “plug in” can anyone tell me what that term means because my mind immediately went to that it sounds like shooting up. If I find out he’s on heroin again I’m going to have to take drastic actions and keep him far away from our daughter he has had TOO many chances