Questions: Relationships

I found out recently that my boyfriend and his ex were still sending each other pictures of NSFW fan art for the first month of our relationship then stopped because they realised it was inappropriate.

For context they were in a long distance relationship and used to send each other NSFW fan art all the time. They broke up when we started dating and she ghosted him for 2 weeks, then when they started talking and hanging out online again they started sending the pictures again.

It's been over a year since this happened and they stopped but they still hang out online together often. I only found out that they used to date recently and then I found out about the pictures and i feel hurt and a bit betrayed.

I want to know if what they did is considered cheating or if it was just really inappropriate.

I'm not sure what to do and having some clarity on this might help me figure it out.

After talking to my boyfriend about it I've forgiven him and he has agreed to stop spending time alone with her and only to talk to her when she's hanging out with our mutual friends but I feel like I should confront her because she kept this from me and befriended me while sending these pictures to my boyfriend and not telling me about their relationship.

If you guys dont consider it cheating then I'll let it go and try to move on but if you do consider it cheating then I feel like I have a right to bring it up despite it being in the past and at the very least demand an apology from her and at worse cut ties with her completely. This whole situation really pains me because I did like her as a friend.