My heart is so heavy

We just found out that my S/O's dad has fathered another child. He and his wife have been married for 20+ years and just celebrated an anniversary in September. My heart is heavy for my child's grandmother as I wouldn't wish this on anyone and we are all completely shocked. Crazy thing is he doesn't seem remorseful at all, I don't even know what to do or how to comfort her. I just can't believe this is happening to her he has ruined the family as we know it