Mum Cucked Me Update

So after speaking to my girlfriend when back after my first university semester, I found out what was happening. At first she wasnt wanting to say anything and was shocked I knew but she came clean eventually
So apparently it had been going on for half a year before, she had come to my house one day to find only my mum was there since it was summer and I was working. My mum let her in and they spoke about many things but apparently my mum was wearing a loose dressing gown and my girlfriend took notice as she is bisexual. When mum would bend over she could see her panty covered arse, my girlfriend said it was the sexiest thing she'd seen and couldn't resist and copped a feel.
My mum apparently flinched when she touched her naked ass and turned round to say something but they just stared them started making out. According to my girlfriend they had met up to fuck multiple times since then and she says their in love with each other. I don't know what to do