Need help in NY - GF wont leave apt

[Marci Stein]

I’m writing this for a friend. 

He rented an apartment and has moved out when lease was up - he didn’t  renew. 

His ex gf  refuses to leave . 

She’s there 1 month , 

the owner is telling my friend it’s up to him to evict her . She won’t pay rent , 

Is that his responsibility? Will he need to pay rent still , even though he returned the keys? ( ex GF made a copy ). 

Any help greatly appreciated .