MR is still the DSM IV diagnosis, so that's what the MD has written. That doesn't make it the appropriate way to refer to the client with whom you are working. Developmentally delayed, or, to use the more preferred "person first" language, person with a developmental delay, are the appropriate ways to refer to your client.
Please, speak to your client. You may not think she understands you, but if her primary caregiver speaks to her and she responds, then she obviously understands. Before performing any care, explain to your client what you're about to do. (i.e.: If she's G-tube fed, explain that you're going to lift her shirt to gain access to her tube, that you're going to open her tube to connect her feeds, etc.) If she requires assistance for feeding, please make sure you identify what she's got to eat, and ask her for her preferences before each bite. It sounds like she's able to communicate in some manner, and before long, you will likely find yourself able to understand her communication efforts.