Wig accessory advice

Hi,
I'm need some advice and I'm hoping someone on this forum has had experience with using wigs on a chemo patient. My friend has lost her hair due to chemo (she has thymic cancer). She has some wigs that she bought but they slip all around her head because there's no hair to hold them in place. I saw a gel headband at an online wig seller that said it would hold the wig in pace on a bald person scalp but I want to ask, does anyone have experience with using this item? Does it stretch out eventually, or cause headaches or rashes?

Also, any advice you can give me on hats would be gratefully accepted. She has 1 knitted cap but I want to get her a softer cotton one for sleeping or wearing around the house as the knit one makes her sweat. I just want to do something for her, because she won't accept any household help and it just kills me to see her try to clean with her fingers that are burning and numb, standing on feet she can't feel except for pins and needles and insisting that she is doing fine. I feel if I can just help her out with the wig issue, I have done something useful for her.

Thank you for any advice -
Terri