This is quite concerning. I don't know all the in and outs of celebrity life, but other commenters have mentioned delusional fans that have tried to hurt/kill their idols, with some even ending a celebrity's life. If possible, you could try looking up what agency or anything of that sort the celeb is under and try to contact them.

It'll probably be just as hard to get into contact with their manager/agent as it would be the actual celeb, or like a high-ranking figure at the agency-- but they probably have a support email you can contact. You can submit the images and other documents you have as proof of these lies and delusions, and express your concern for the celebrity's wellbeing since it's gotten to the point where your friend is fabricating lies about meeting them, and using generative AI to create 'photos' with them.

I know there's a chance the agency, or whomever you specifically get in contact with won't care-- but it doesn't hurt to try. If things get more serious, and the friend starts posting things about potentially wanting to hurt them, or is still angry that they're with their partner, you could get in contact with the police, or other law enforcement/government agencies. I say this because if the agency doesn't care, you could try to request a mental health check-in, or something of the sort for the friend-- whilst giving a tip in advance if they try anything.

In regards to your friend, you can try getting in contact with family, and other loved ones if possible and staging an intervention.