Sexting with a good friend while in relationship

Hey,I caught my BF of 5 years (we're both 30) sexting with an old friend of his. My BF knows this friend from many years ago. They also live in different continents (we are in Europe and the friend is in the US) so there's not danger of them meeting and they mostly communicate on Facebook.Recently we traveled to the US and almost met this friend for a dinner. So they are quite close.I was quite upset but we talked about building trust again and trying to understand the underlying issues we have in the relationship (we still have more work to do).The fact that my BF and his friend are still talking (daily or weekly) bothers me. I asked my BF to delete him and stop any communication with him. My BF agreed but was very sad about that. I also felt bad for making him loosing an old friend for something that we are currently working to fix. So my request is on hold until I make up my mind towards his friend.Assuming that my BF and his friend won't sext again (and if they do than it's a different topic), am I over-reacting for asking my BF to stop communicating with that guy?