Friends husband won't give up

My friends husband keeps messaging me on Instagram. I'm married and he knows that. He is also married to my good friend which he isn't respecting. He sends me nudes out of nowhere, calls me, texts me all day, and tells me to "Answer." I've already told him from the get-go to stop speaking to me this way because I respect my friend, and he should respect his wife and our friendship. I am now debating to let my friend know or just don't get involved. I hate drama and I'd hate to ruin her marriage and our friendship. I told my husband about it and he says don't tell her anything. I am pretty sure this guy is talking to plenty of other women so I think she will find out for herself he isn't a loyal guy. He was in prison from age 18 until 34 for being a pedophile/rapist. She knows his history anyway but maybe she thought he would change by marrying her. Idk.. but the way he calls me and demands me to respond is pretty scary. I never had someone demand a response like this. I am 27 but look younger and he is now 41. Not that it matters but maybe it's the rapist in him that won't give up?