I'm so sorry to hear about your student's mom...I can't even imagine battling breast cancer twice
I hope that she's feeling better soon and that she can have a permanent remission.
How kind of you to make her a hat!
site has some cute patterns for crocheted, knitted, and sewed chemo caps. For fibers, they recommend softer yarns like cotton and state them to be more comfortable. A yarn they mention by name brand is Lion Brand's Microspun because it's soft and machine washable and dryable, and it's acrylic (no allergies). They state the fun furs are soft and lightweight, and that thinner, seamless, and non-lumpy caps "are good for sleeping and increased comfort", and that "hats made with thinner yarn would be better for summer weather use".
Seems like softness, breathability, and lightweight are all factors for chemo caps when considering what type of yarn to use.