Both the double hung windows have something wrong with them The parting rails don't meet on either window. It's obvious in the other window. And can be seen in the one that Fraser closes after deciding not to jump.
While Fraiser is on the ledge and after Lilith tells him that she can't lie to him, we see a shot of Frasier on the ledge. Between his legs, you can see the safety tether which is protecting him from falling out the window.