This is a long one, so buckle up. Apologies in advance.
I have been with my partner for 6 years. We now have a 5m/o son. I have a rocky understanding of his family dynamic and have done for some time. A few months ago, my FIL approached my partner and informed him he wants to divorce my MIL. My FIL had not told anyone how he was feeling, including my MIL. My FIL’s rationale was that he wanted to prepare my partner to help “look after” my MIL. However, he reported he’d felt this way for 6+ years.
Fast forward about a month, my FIL told my MIL how he was feeling. It obviously didn’t go down well but they continued to stay in the same house (separate bedrooms) My MIL is not independent, hasn’t worked in 10+ years etc. MIL was hung up on wanting to just be around him, even if not in a romantic relationship with him.
Then, out of the blue, my partner was told by my SIL (who had been told about this all by the woman herself) that my FIL had been having romantic conversations with another woman - who just so happens to be my MIL’s cousin and her only friend (We’ll call her Sarah). Sarah recently went through a divorce of her own. Sarah and my FIL bumped in to one another whilst shopping and apparently it went from there. FIL was asked to leave by my partners brother. FIL has recently remortgaged and has been caught out lying to us about using the house to let my little one play.
My partner is destroyed - he feels so hurt and betrayed but mainly angry at Sarah. This is a woman we were very close to, she was one of the first people to know about my pregnancy, she’s recently spent time at our house - asking how my MIL is coping and that she is “worried about her.” He’s not very good at being open with his emotions and asking for help. I want to help and support him but I don’t want to get too involved as this situation completely changes their whole family dynamic. I feel a certain way about the situation but I know it’s not my place to express that and express my anger because he isn’t my dad and it isn’t my direct family.
How can I support my partner whilst trying to remain as indirectly involved as possible?