Certainly given what your mom has been through, she is going to have difficulty with her mouth, it is sore, and she likely has some trismus from the radiation treatments which make it difficult (not painful), to open her mouth fully. You and/or her doctors should ask her exactly what it is that she is experiencing. If it is the mucositis and sores from radiation, this is only normal after the procedure and will go away given time. Remember that she also has thick saliva if any at all, and other issues related to eating and swallowing. All this is normal if she is just coming out of treatment. If it is difficulty in opening her mouth, she needs to work daily on stretching the muscles. The device mentioned in the previous posts is one way to accomplish this. But before you decide that something must be done, you need to ask her exactly what she is experiencing.