I am very glad your Mum got through surgery but so sorry about the complications and subsequent pain.
I do hope she is given adequate pain meds and if not, do keep asking. I did have a lot for the first week while in hospital and was able to reduce them before leaving at 11 days.
I used Xylocaine Viscous after radiotherapy to numb my mouth, not only to allow for relatively comfortable eating but also for general relief at any time. After swirling/holding in the mouth for a short while, it can then be safely swallowed, numbing the throat on the way down as well. Please ask her docs for any options. Unless you point out strongly the need for relief, it can be easily overlooked.
Prayers for good blessings as needed, (((hugs))) to you and your Mum, Pamela.