I went and see Gloria's post, but I'm not sure about the meaning of the original sentence as happened to her. I think the meaning was "He helped her to get into his car"
When you translate it into Spanish you can have this problem since "su" can mean "his", "her" , "their" and even "your" if you are using the form "usted" (polite "you" in Spain)
"La ayudó a subir a su coche y la llevó a su casa"
In the last sentence you can't know if it's her or his car, neither if we're referring to his or her house.
We could say:
"Él la ayudó a subir al coche de ella"
(He helped her to get into her car)
If you sayd:
"Él la ayudó a subir en su coche"
I'd understand it's his car, though it could also mean mean her car, so I'd simply use this last sentence.
I hope it helps.