WTF!!!!

If it was me, I would definitely be concerned about whether or not the friend with cancer still wanted, or could be, a bridesmaid. Not because of a "look" but because it is a horrible disease that takes a lot out of a person. I would want them to know that I would love for them to be bridesmaid but that it was ok for them to step down if they needed to - that I would understand. That I just wanted to make sure that they were ok. And if they were still up to it then I would ask them what they wanted to do about the hair thing - maybe go shopping with them to find material for a beautiful head wrap if that's what they wanted. I would first make sure that they were happy and comfortable and got whatever they needed. Heck I would probably see if all the bridesmaid were ok with wearing head wrap so that she wouldn't stand out (if she didn't want to draw too much attention to herself).

I would also potentially see if sometime would be willing to be a backup bridesmaid in case she couldn't make it last minute or speak to the groomsmen that she is supposed to be paired with to see if he was ok with either being a lone groomsmen or standing down last minute.

Being a bridesmaid is a lot of work, even when it is just the day of and can be tiring for a healthy person - basically I would just want make sure that my friend knew that I would not be angry or hold it against her of she just didn't feel up to it. That she didn't need to be worried about that.