Advise please

So when has it been ok for your boyfriend/girlfriend to call on their ex for a favor while being in a whole relationship? With that being said my boyfriend thinks that s**t is ok as far as him being able to drive his ex car, her brother coming to give him a ride to work and i clearly have a car. Her calling him all in her feelings cause he not answering his phone for her so i told him i believe him and his ex got something going on. Now ladies what is considered crossing the line to you?