What should I do about my brother's friend's girlfriend blocking me on Facebook?

On behalf of a female friend:

My brother's friend Peter who I have known since he was 10 has recently been persuaded by his girlfriend to block me on facebook. We often play board games in sibling groups and have a perfectly innocent friendship. We only ever meet in the presence of my brother, and sometimes Peter's brother as well.

This is concerning to me as I mostly just update him on whether my brother Sammy is awake, and ask for advice on games like My Singing Monsters. I now have to rely on Wikipedia, which is a lot less reliable. Also, without me waking Sammy up to get him to answer the phone, he and Peter would never meet socially.

It had been previously mentioned by my brother that Peter's girlfriend told him she was getting ''well annoyed with your sister keep sending Peter messages''. Peter has kept secrets from her in the past, such as coming to our house to play Dungeons and Dragons. Around this time there could have been messages about me being a 'Dungeon Master' which could be misinterpreted. Or maybe he wants to keep his geek side hidden. She really hates my brother (she actually banned Peter from seeing him for one whole year).

I do not often send messages - the last conversation we had was 6 months ago (should my brother buy a desktop computer or not).

I have never met this girl, wanted to invite her over with Peter. My brother says she's bound to hate me as much as she hates him (though we are quite different) and probably won't come anyway.

Thanks!