What do you think about me  reporting my flatmate to the housing agency that she lives here with her boyfriend when it's forbidden?
Why is your flatmate's boyfriend a problem?
Everyone in the building are fed up with her boyfriend stuffing in his earphones, blaring out horrible music for 24/7.
Someone that isn't on the lease legally isn't considered a tenant. You should find out what your rights are and make sure that you are not causing damage to your flatmate unreasonably.
It's not my problem. I'm not affected by his presence, so why should I care.
If you have a problem with your flatmate's boyfriend then you should talk to her about it. However, if you don't have a problem with him then you should not get involved.