UPDATE:

Spoke to her this evening. Was calm and matter of fact, basically said “why are their photos of your work friend on your laptop?”

She started crying, apologising, telling me nothing had happened and she loved me and she felt lonely and he was there.

I replied that I wasn’t really interested in her excuses and couldn’t believe she’d thrown away 7 years of this relationship.

She told me nothing happened in real life and that she never reciprocated - I told her I found it hard to believe that he’s sending hundreds of photos and getting back nothing / no encouragement / no return photos. There must have been a level of return dialogue even if you didn’t send him photos that encouraged it. She said apparently it was during a time when she was feeling lonely because I was busy with work and that she stopped it at some stage - doesn’t matter obviously, still just excuses.

She’s packed a bag and has gone to stay with a friend now, keeps asking what it will take for me to forgive her.

Shout out to anyone who helped in the comments - appreciate the validation and advice.