Message your friend separately and tell them that what they’re doing isn’t okay. You don't want your friend to break the law and do something he will regret. Tell them it's illegal and they should delete the photos.
 

If your friend refuses to delete the photo, you could report them anonymously.

It’s not easy to report a friend but it’s important to remember that his behaviour is unacceptable, illegal and could have a lasting impact on his ex. You can contact the Revenge Porn Helpline on 03456 000 459, or by email.
 

If your friend refuses to delete the photo, you could tell his ex what he’s doing and offer to report it.

It might be hard to go behind your friend’s back but what he’s doing could affect his ex for the rest of her life. By letting her know what he’s doing and offering to share the evidence if she wants to report it, you’re supporting a victim of intimate image abuse.
 

Changing the subject in the group chat could stop the situation from escalating, but sharing nude photos of someone else without their consent is illegal, and ignoring the messages could be seen as condoning the behaviour. You don’t want your friend to break the law and do something he will regret, so there might be a better way to intervene safely.