I imagine it's different for everyone, but with us, when he was seeing a T, he didn't run home to talk about it. Occasionally, he would bring something up, but I never asked him any questions unless he opened the topic himself. This worked for us, because there was a "3 comment rule". If he brought up the same thing 3 times then it was okay to ask and explore the issue. (It's the truth, but as I write this, I can't help but laugh at what a "conversational dance" we do) And, even after all these years, if I ask a question too soon, all bets are off.
This doesn't just apply to therapy, this applies to anything involving him, his work, how he's feeling etc. Sometimes, if it gets to the point where I'm actually tripping over the "800 pound gorilla in the room", I might pose a question like, "Are you going to tell me what's bothering you?" Sometimes it works, sometimes it doesn't, but the purpose more or less is to let him know that clearly something is bothering him and, yes, I've noticed.
Hopefully, the therapist will ask the necessary questions and your friend will answer if and when he can. The best thing you can do is just be there for him should he need to run something past you, which does happen.
Best of luck to you both.