It's just hair
Treatments like chemotherapy can take a toll on someone's appearance. The first priority is to get rid of the cancerous cells but, it's also hard seeing all your hair fall out.
This physical change can be hard for both men and women and although you're trying to be supportive, you shouldn't minimize what they're going through by saying that it's just hair.
This change can affect someone's self-esteem so avoid saying this.
Be happy
Telling someone going through grueling cancer treatments to be happy is like telling someone struggling with depression to just stop being depressed. Saying this is anything but discouraging and it can make your loved one feel worse.
You know that what they're going through isn't easy. That is why you should probably find more appropriate words that can cheer them up.
Be strong
This is another sentence that you should avoid at all times. Yes, the changes they're going through requires them to be strong, but they also need to know that they are worthy of support.
Maybe previously they would handle every challenge that came their way but now, they shouldn't feel forced to maintain that strong image.
Cancer is no joke and if you've never had that experience, it's easier to tell someone else to just be strong, which doesn't work. Give them room to be vulnerable and don't pressure them to be strong.
Have you tried traditional medicine?
This one here can be a bit controversial because many people would be open to alternative forms of medicine. However, many of these suggestions don't work and these remedies can even make someone who already has many complications sicker or even die.
Also, these comments can be quite offensive to some people. How sure are you they haven't tried every treatment in the book without much success? Or, if they hold the same medical beliefs as you do?
The best thing to do is let the doctor be in charge because what you're suggesting can be detrimental to their health.