Is it possible he started late with her for whatever reason? So maybe he's just giving her the usual 50 minutes, but didn't start until 10 after the hour? I'd definitely talk to him about it if it bothers you.

My individual T is very punctual, but my marriage counselor is usually late--I'm just used to it at this point. He also gives us extra time sometimes, so I figure it's OK if he gives it to others on occasion (and I think he tends to run late in getting to the office, so sometimes it's just that). But I definitely understand if it seems like he's always giving that one client before you extra time...