While I've never had an OW in my R, I've never been separated in another place. Being physically separated kinda opens some doors that need not be opened unless a marriage has ended, I think. So, I am not sure if being angry about him acting out during a separation is where your energy needs to lay just now.
Letting his parents know that you hope for the R and are ready to work for it (not just because of the baby) is fair. It is their baby, too as grandparents. Why not let them know that you found out the separation had led to the OW? As long as it isn't vindictive...
