If you've watched the episode, Phoebe is at the bar and Ross arrives moments later, informing her of Susan being a lesbian. The entire bar since the start of this scene is empty! There's not even a bartender as Ross proceeds to make himself several drinks to drown his sorrow. What kind of a 'buffay' (buffet) bar is this? :P
OK, so it seems you watched an edited version of this episode. This is explained in the uncut version:
[Scene: the bar, Ross is entering, Phoebe is at the bar, they are the only two in the place.]
Ross: (all depressed) Hi. Where is everybody?
Phoebe: Oh, it’s already closed, Chris gave me the keys to lock up-what is wrong?
It seems the guys frequent the bar so much that the owner or manager trusts them (plus Phoebe is very honest and friendly). And the bar was about to be closed down indefinitely anyway and turned into a coffee shop, so maybe he just didn't care anymore. ;)