Say something or stay out of it

My step son is 13 and he was complaining about his mom to my husband. She's not a good person and has put him through a lot. He has more trauma than I could ever imagine. They spoke on the phone and it ended up him really mad and came to talk to my husband and he was saying "I fucking hate Diana!" He calls her by her first name. My husband is on his phone while he's talking about the things his mom said and just saying mmhmm. My husband gets annoyed with him complaining about his mom and believes that is for his therapist to work through. I don't have biological kids but I do feel if your kid is upset you should listen. He picked up his dad wasn't listening and said nobody ever listens to him and slammed his door and my husband just rolled his eyes. This feels mean. She put him through some really fucked up shit and my husband wasn't perfect because he left. My husband always says "I didn't leave you. I left your mother." And I do tell my husband he has to understand his POV because he left him in all of that. He knew how his mom is and chose to leave his child in that so he can feel some type of way. I don't have biological kids so maybe I have no idea what I'm talking about.