It sounds like the SAHD was talking to you just like a girlfriend. Just the way any women would talk about her marriage problems and / or concerns.
I have had this happen a couple of times in my life where I was coordinating the playdates with the dads. I never felt it became inappropriate, but at first, to be very honest with you, I was not used to discussing such problems with guys. As I relaxed and got to know the guy(s) better, yes they wanted to talk about stuff just like any girlfriend.
Burt what I liked better, is that guys are more problem solving, solution oriented. So when I gave advice, they actually listened and tried it. Whereas a women just really wanted to complain and vent and not improve anything. So, over time, it was rather nice hanging out with the guys. We shared recipes and coupons, etc.
One of the men, we ended up having surgeries at the same time and became very helpful with each other's PT appointments and grocery shopping, etc. I could drive and he could not, but he could walk and I could not.
I just want to say, that if a girl admitted she was lonely, you'd probably react appropriately with girl advice. Unless you felt the guy was coming onto you and it felt inappropriate, he's human and is having the same marriage struggles we all have to contend with.
GL!