In Pennsylvania, is it permitted to gain employment at a bar while on dui probation?

Probably not a good idea to do this. I wouldn't even think about it.

Is there not a restriction on entering places that serve drinks?

But he can check with his PO

I think there is no law against it, if you are apply for a bartender position. There's probably a background check for bouncers. However, it is probably not in your best interest if you wish to avoid further convictions for DUI - if you don't have the willpower to drink and not drive ONCE, do you really think this time you can make a good decision when impared?