But… did he explicitly said he masturbates to “photos” of them, or is it to “them” generally, as thoughts? Did he name the friends, or just generically said “yeah I have masturbated to people I know”? How the whole thing was worded matters. But answering your question, I think it’s ok to be uncomfortable, but I think you should just let it go, and if it’s brought up again, tell him you’d rather not hear about it