Two pieces of potentially useful information:
"You look great!!"
I know that this is intended as a supportive statement, but please don't say this to a person with a life threatening or serious chronic illness, unless you know that how they look is something they - as an individual - particularly care about.
Now, if they just got a new outfit, or a new wig, then fine, compliment them on the new fashion attire.
But generally speaking, a person who is currently in great pain, or struggling with nausea, or looking at their last few months of life... that person is unlikely to get too excited about how he or she looks. Unless they are concerned about their looks for specific reasons, or unless you know them to be a person who is consistently very concerned with appearance, the seriously ill person is likely to interpret your statement as a denial of their illness. It can seem to them that you either don't want to believe that they are truly ill, or that you want to keep them from talking about or showing any signs of illness while you are with them.
Neither interpretation reflects well on you, and it puts the ill person on the spot, forcing them to come up with a tactful reply and to try to conform to your needs without really knowing what those needs are. People who are seriously ill have enough on their minds without having to deal with that sort of thing. Do them (and yourself) a favor, and instead either compliment them on a specific thing - the color of the new outfit, the attractiveness of the new haircut, the cut of the new outfit, the color of the new hair - or present a different subject of conversation. You'll be doing everyone a favor.
That said, the second bit of information is about looking good.
Most forms of chemo treatment affect your hair. There are also non-chemo drug treatments that thin your hair or cause it to fall out in patches (as well as causing very unpleasant scalp/skin conditions, but that's a different story). It's all, as they say, No Fun.
There are stores and websites at which you can buy various types of hats and wigs and other types of head coverings. The American Cancer Society, for instance, has an online store and catalog that has very reasonably priced options. But today I want to bring to your attention two particular online sites.
One is heavenlyhats.com, a charitable organization started by a thoughtful young man inspired by the plight of pediatric cancer patients in a local hospital. This organization will send out free hats to cancer patients and folks with other medical conditions that affect hair growth - Male Pattern Baldness does not qualify, sorry. If you need hats, go apply. If you don't need hats, but you'd like to donate to a good cause, go there and give.
The other is France Luxe, a website store that sells a wide variety of hair adornments - barrettes, headbands, scarves, turbans, jeweled hair sticks, etc. The lovely CEO of France Luxe, along with her employees, will send cancer patients one free silk or cotton headwrap of their choice. All you have to do is go to the website, click on the little 'Good Wishes' icon at the top of the home page, and you will find the application. This is a very generous thing for them to do - so if you have a wedding or prom coming up, or if you are hankering for something pretty for your hair, please check them out and see if you can support them by buying something nice for yourself. If you go here and buy products from this page, 20% of your purchase will go to the Triple Negative Breast Cancer Foundation. Other than chemo, there are currently few treatment options for those with triple negative cancer. Anything developed that would cure or control triple negative cancer will also cure or control the other sorts, too, so it's a good cause for all.