I'm 5'4", and I don't have any problems getting in my truck and neither does my girlfriend... she's short, too. I even have a bad knee from being in the Army.
My solution? Teach her how to get inside. I've had to teach all my girlfriends how to get into my trucks so they don't fall and hurt themselves, especially my offroad truck which is somewhat tall and the rockers are slippery if they're wet or muddy.
I haven't had anyone complain after they pay attention to how to get in correctly.