M.G.
There are waterproof sprays for fabric. Just check a fabric store, Walmart, Amazon ... or ask at a website for this kind of thing (knitting or crocheting or sewing - quilting). I'm sure you'd get some advice.
I used to go to a fabric store years ago, and the ladies there were a wonderful source of knowledge.
Otherwise, I'd just suggest they use it with placemats. We use cotton tablecloths and just use stain remover to get out stains. Works well. Sounds like something you'd use now and then - not for regular meals? Is it for special occasions? If used regularly - then expects some spills. It's life with four kids I'm sure. Sprays tend to come off with regular washings, so she'd have to reapply most likely.