Hi Bethan , thanks for posting this question and I'm sorry to hear of your mum's diagnosis. As we wait for others to share any experiences with coeliac and chemotherapy, I wanted to share the below resource below that might possibly be helpful for your mum. Have you had a chance to touch base with a dietition or nutritionist as they may be able to offer some helpful tailored tips for gut health?
BCNA Podcast - Nutrition & breast cancer has some helpful advice on a ketogenic diet which can be adapted for coeliac needs (please consult your healthcare team).