I don't know - I think it is a completely unique situation every single time. I mean, if I was scanning my sister's company's website and I realized that the small white print on the black background made it really impossible to read the words, I would fire her a text that said something like, "I was looking at your website and the white text with black background is really hard for your old sister to read. You might want to consider changing that." I'd stay away from "you should".
But, if my sister was mad at me for that because in her mind it was obvious that it was a problem and she planned on fixing it eventually, I think that my sister would be out of line. Because how would I know that she thought that? I mean, if she looked at her website and realized it was unreadable, I would think she would change it right away and if it hasn't been changed I will assume that she doesn't see the issue.
However, it is unclear to me what your scenario actually is. If your website is a complete disaster visually and it's been that way for a while, I'm not surprised that a friend has come to you and said something. Because they probably think you don't realize it or you would have changed it by now. But, that may or may not be your situation