I sorry to hear that I wish your friend was little more understanding....a really good friend would not make you decide beween friendship and an illness. I would hate for you get so upset with self if you cant keep your promise just I would hate to hear that you lots a friend over it. I know how hard is deal with people who dont understand...I'm just a little for stubborn I tend to brack up a friendship like that before thay can do it do me cause I cant handle the expection to be normal.
In my oppionion if her mum gives you the 3rd degree tell her to go talk to a profession about why people SI because you dont need the stress of dealing with 3rd degree. Tell your friend and her mum to do there own reserch about people who SI then if they still cant be understanding then they may not be the right people to be around too often you deserve to be around people who can be understanding. One of the reasons I only have one friend right now and can ignore my OCD thought to ruin the friendship is because my current friend know I SI she knows about lots of my issues and while she does not understand she is understanding she accepts me for me and does not expect anything but friendship from me. Fun, shopping, coffee and chat, shoulder to cry on ect ect....she has never in any way excpected more from me than i can give. I sure could not promise to not SI I can just promise that I will try ignore the SI thoughts.

Take care and I really would just write a letter or tell you friend and her mum to talk to a professional and/or do reserch of there own. Focus on being positive and dont let this get to you to much you deserve to feel relaxed and not stressed out about whats might happen next.