I would wonder why this ex is keeping tabs on PnQ's. It was a very brief less than a month, couple of dates kind of thing, certainly not a long history of d@ting or even having been friends before hand.
OP, I'm glad you've blocked him
I would hardly call his behavior stalkerish. It's not like he had to go out in search of information on Peas; they were FB friends. If Peas didn't want him to see her posts she just had to de-friend him.
I honestly don't think the guy did anything wrong in responding to a post that more than likely showed up on his feed. And if Peas doesn't want him privy to her information the right thing to do is de-friend and block.
Agreed, and if you're going to call him stalker-ish for doing something completely with in the realm of normal for that medium, what does that make PnQ's for friending him after such a casual, short, relationshp in the first place? It goes both ways.
OP, I'm glad you've resolved this issue in a way that's satisfactory for you.