I need anyones opinion

So I have a friend she’s in a relationship with this guy for 5 months she has told me that she has been feeling depressed and feeling like not herself and this depression has been going on for a while now and one day her guy friend asked her to go smoke they smoked she got stoned and he was as well and they did it and she regret it because she knew although her bf wasn’t showing all his love because he was busy with school and taking care of his daughter she knew it was wrong she told me she doesn’t wanna tell her bf what happened because she doesn’t want to lose him,she never intended for that to happen but it did and honestly I don’t know what to tell her I know it’s bad what happened but she does really love him she does anything for her bf just idk what’s your guys opinion what should I tell her ?