sorry your Mum is having that trouble. I had the same when I first went on oxygen, I was advised KY jelly but it was useless. I bought some Nasogel by Neilmed, its safe to use with oxygen and helps a lot. However I also purchase some really soft cannulae from the USA and that has completely solved my problem. The company's address is
they are very helpful, you can e-mail them if you need any advice, they post out very quickly.
Someone has mentioned bubbling the oxygen through a water bottle and some people do use this method, I am not on a high enough prescription for this, don't know how many litres your Mum is on. The BLF nurses may be able to advise, otherwise get on to the oxygen suppliers, they are usually very helpful.
Your Mum must be glad that she has you looking out for her, I do hope that she gets some relief soon.