I don't know what the rules are in Holland. Here in the US professionals are told to avoid relationships like Facebook and the like with clients or former clients. It opens the door for a whole set of possible ethical problems.

As for seeing the person face-to-face on the street. Again, I don't know the rules in Holland, but here in the US the professional should follow the lead of the client. If the client approaches and talks to them it's okay to respond. If the client acts as if they don't know the professional, they should also act like they don't know the client.

Does taht help?