Mum has left Dad - From Anonymous

I would like to share my story but please keep my name anonymous.
My Mum is in prison and has just sent my Dad a letter saying she has left him for another woman who is also in prison. 
I am so upset words cannot describe the way I feel. Dad has stood by her and supported her and she treats him like this!!! 
I don't have any support and I will not be visiting my Mum again. I don't even want to speak to her. Is there any more teenagers out there who have experienced this? I feel so alone and Dad is in pieces. I don't know what to say to him because nothing will make him feel better. Thank you.