My dad (70) just retired and now he wants to divorce my mom (65).

My dad just retired, I was excited for him to start finally doing the things he enjoys. Apparently his plans for retirement were a lot different than I could have imagined. Just the other day he called me after I got home from work, he said he isn't happy with my mom and "can't do this anymore". He also said he's been talking to his ex-wife from about 40 years ago, who he divorced because she cheated on him. He says he thinks they might get back together.
My mom and dad have never had the greatest relationship, I will not deny that, but I always assumed it worked for them somehow, and they would stay together. Of all times to decide this isn't what he wants, at 70? After retirement? He won't even consider marriage counseling. It all seems completely insane.
My siblings are not handling it well. My one sister is trying to play counselor and get them to fix things, but I just don't see that working. My other sister is hysterical and just calls my dad to scream at him. My brother has lots of emotional problems and still lives with them, and I really worry about how he'll deal with this.
To top it all off my boyfriend just proposed a week ago, and we set a wedding date for this year, only I can't imagine how that will go now...
I don't know what to do, or if there's anything I can reasonably do.
TL;DR My dad wants to divorce my mom, and possibly get back with his ex after he just turned 70 and retired. None of my siblings are handling it well and I don't know what if anything I can do.


Source.