I would recommend practicing this with her. When she is lying on her back cross one of her legs over the other and the same side arm across her chest and slightly push the bottom half of her body encouraging her to turn. I had a 1 year old with the same issue and I practiced this with him every diaper change and a few times in between and he progressed in no time.
I would recommend checking to see if she seems to have any decreased muscle tone or strength. Although this is not our job, sometimes parents really do not know what appears to be normal development and what actually might be a potential issue. Does she sit in a W position, can she bear weight through both legs equally while standing, does she seem to have normal strenght in all her limbs?
The not rolling thing, although odd, can likely be addressed easily with a bit of practice, however her feeling like dead weight when she is being carried seems very odd to me.
How are the parents dealing with these issues? Are they aware of them? Did they notice the progress she is making since she is with you?
Although these are difficult things to discuss with parents, especially when the parent seems oblivious, if the child requires a medical assessment or intervention, it would be good to discuss sooner rather than later. My d/c kid also had issues with his stance (bending forward at the waist) and his walking (internal rotation of the feet and walking on the outer part of the foot), the parents chose to bring him to a chiro (although this is really not the route I would've taken)... but he improved alot with adjustments. The parents had not noticed any issues before I started discussing these with them.
Good luck