I was on about 20 mg of amitriptyline for a chronic pain condition for about 5 years. I took it at night about an hour before bed, as it does make you groggy.
I got a dry throat, but not any other side effects I can remember.
For a short while I went up to 50mg for depression and I did feel like a zombie and like I was floating and my feet weren't touching the ground, but i would say that is just a short term (weeks) side affect of the body adjusting to the medication. I choose to go back to my 20 mg dose quite soon.
good luck for you mum. It definitely helped me to get restorative REM sleep and boost my serotonin therefore helping chronic pain. It also helped my stress to fade a little.
After about 5 years, I changed to nortriptyline on a similar dose, which suited me better - so keep that in mind for the future. It had less side effects for me and is in the same 'family' of drugs, and used for similar things.
If your mum is struggling, she could try halving the tablet for the first week or so, but probably check this with her gp first.