IF you do end up choosing to go with a plan, think it through carefully. As carefully as possible. Do not simply tell her mother. I would plan beforehand and think about what exactly you will say to her and how you will respond to the various reactions that might come of it. This is a complex situation and you need backup plans. I'm not going to go into all of the possibilities as I do not fully understand the situation and I am not an expert at all in these matters. Since you care about the safety of your friend who is unpredictable and emotionally unstable, you must ensure that the mother will act rationally. Depending on the mother, she might even forgive her daughter, as often the maternal bond trumps various emotions and other factors that take place in conflicts. There is a chance that jessica will be very affected by this situation and won't deal with it conventionally. she might take drastic measures or disconnect herself from people once she has realized the reality of her relationship with M in combination with her altered relationship with her mother. I do not know Jessica personally, but do consider a psych ward. This protects her mostly from herself and gives her time to heal and self-reflect without damaging herself. This might sound drastic but just consider it. I hope things go well, but please do think carefully. I suggest you create potential solutions and consequences and try to pan them out so you are able to help but leave the situation as effectively as possible with the best result.