Should I send a letter

I caught my partner of 8.5 years cheating a year ago. We have tried to move on from that, but recently I discovered he has also been hiding a female friend, he has been seeing her the whole time we've been together but says they are "just friends". I sent her a message when her sister died saying I was sorry to hear of her loss and told her I was "Ted's" wife. She didn't respond and blocked me. I thought this was a strange reaction to my message. Ted has admitted that he hide me from her and her from me but I just have this gut feeling that it's not just friends. He said she doesn't want anything to do with me because of prior relationships he's had that caused her trouble. He also told me that whenever one of his relationships failed he would go and live with her until the next one came along. I can't get over how well he hid this from me and I'm sure I still wouldn't know about it unless I investigated things due to his affair last year. Should I mail her a letter and tell her that we've been living together for 7 years
and no matter what he's told her about me it's a lie. I think when I sent that message he told her I was just his landlady. He's a very good liar to be sure. I don't want to make a mistake and just kick him out, but I also think she needs to know the situation. What do I do?