Friend becoming a stalker.

I've known my best friend's boyfriend for a year now. They seperated about 3 months ago but its not a definite break. Since he suffers from a lifethreatening illness and I live close by, my best friend asked me to keep an eye on him. I chatted with him every night and he brought me some gifts. He told me the gifts were to say thank you for a period of time I had supported him and his girlfriend when she had lost her job. With my best friend in another town, his attitude towards me changed. He now shows up uninvited every day. I'm at the point where i'm hiding when i see him. The gifts have turned to lingerie and the night chats he turned into sexual topics. When he 'accidently' sent me a photo of his private parts, I stopped all contact. That was about three weeks ago. But he still tries to contact me on all my chat apps, phone numbers, my father's phone, stops by my house every day and has even told his girlfriend/my best friend that I'm no longer talking to him. I feel that its him trying another pressure tactic. I'm unfortunately not the straight-talking type, I'm a very introverted loner. So I cannot tell him straight to buzz off because I know him, he will pretend I'm over reacting. I don't want to tell my best friend whats been happening either because its so embarresing.

1. Why didn't you report his inappropriate behaviour to your best friend the minute it started? 2. How come before then you didn't phone girl friend to thank her for the joint gifts? 3. How come girlfriend, on hearing that you'd cut all contact, wasn't immediately on the phone to you to ask why? 4. In what way is his misbehaviour your embarrassment?