there's this one reeeeeaaaaallllyy ditzy girl at my job who im training to.close when i go on leave…

there's this one reeeeeaaaaallllyy ditzy girl at my job who im training to.close when i go on leave and she cant even take orders and deal with a customer at the window at the same time and shes super sweet but I can only tale her stupidity for about 2 hours before i snap on her, then I feel bad. so one time like 6 people called and complained on her in a row and i told her and her response is "I wasn't rude to anyone" like they wouldn't call if you weren't. she always has an explanation for everything like just be quiet and take this constructive criticism. those customers are gonna eat her alive when i leave. and i dont feel sorry for her. she irks me so bad. but she's so nice and i feel bad for being so annoyed with her