Here, Mason is someone's name, glass is the kind of glass from a window, and the car seat is a child safety seat.
It is not a well-established idiom. It's a line from the show 13 Reasons Why
Kevin Porter: [to his wife] This silent treatment gonna last all day?
Tracy Porter: What do you want me to say, Kev? I was glad to bail you out of jail and wait 2 hours at the impound?
Kevin Porter: You said that I should use the time that I have left to do the most good. Take care of those kids.
Tracy Porter: And how is assaulting a drug dealer taking care of those kids?
Kevin Porter: He all but put a price tag on Justin Foley's head.
Tracy Porter: And what happened to the back window, hmm?
Kevin Porter: It... it must have happened during the fight. I mean, we'll... we'll get it fixed.
Tracy Porter: It's not about the car. It...
Tracy Porter: Look, I'm proud of you, Kev. Fighting for these kids, doing whatever you can, I get it. But, baby, there's glass in Mason's car seat. I want you to do you, but what if someone from the school finds out what happened and you get fired?
Kevin Porter: They won't, all right?
Tracy Porter: And what if you get hurt next time?
Kevin Porter: There won't be a next time. I promise.
I can't exactly tell you what your friend intended, but it seems to me like he was trying to be empathetic and at the same time warn you that you shouldn't have been doing that--that there are consequences.
If you're actually asking about the TV dialogue, then there is no idiom. It was literal. There was glass from the window in Mason's child safety seat. The speaker was simply pointing out the consequences of Kevin's actions. The window got smashed in a fight and now his career is in jeopardy. These are the consequences of his doing what he thinks is "the most good."