Using the grey scale to determine value is an excellent idea. That's how I taught value to young children in art classes. I also taught them to squint at a picture (insert "fabrics" ) to see which ones are light, medium and dark. I called them "the 3 bears" to help them remember the concept. The 3 bears works well for picking patterned fabrics too.

the easiest way to see color value is to use a piece of red film they used to make the old 3D glasses out of - it turns everything in to shades of reddish-gray and then she will see what you mean. Some of your quilting templates are that color and can be laid down on fabric to see what I am talking about. Or if you have some of those 3D glasses, just look out of the red side. As for color choices - buy a peep hole that you would put in a front door and look backwards through it. I have a child's kalediscope that I use to "see" what 1 square looks like as multiples. It helps a lot, too.

I was also going to suggest that you tell her about the Bonnie Hunter site. She needs to spend some time reading and digesting all her information.

And my reaction to your friend's interesting color combo on the 9 patch...WOW!

My LQS has a sheet of red transparent plastic (like the weight of pattern plastic. When customers picks out colors, they often use this red plastic to mask the colors and focus on the values. Maybe that would help.

as a colorblind woman i must tell you that most of us don't know we're colorblind.we , most i'm sure, see color but can't always discern one from another. my family is very helpful if i ask, but otherwise say nothing and chalk it up to my 'artistic temperment and creativity'. eye drs rarely believe that a woman they're testing is actually color blind. they don't even look for it most times because it is so rare in women. so it's very possible that your friend is either colorblind, or 'color ignorant'. color ignorant you can teach. color blind you can't.

You all could be talking about me (and I'm not color blind). (Although I do not have a stash from the 70's).
I can coordinate my clothing but have a heck of a time trying to make fabrics work together. I can do colors, but not scale. That is why I will often copy the colors in a quilt.
I always take a friend along who has been quilting for a long time. As far as a book...I would never slow down long enough to take the time to read it. I would rather be quilting..so I am happy copying and occasionally do a good job picking out colors... I am pretty new at this and am still learning.
I just appreciate any help that I get.. Lend your friend a pair of scissors and offer to help chop that orange fabric smaller (However, I do like a soft coral or orangey color--lol).

I use the 3-1 color tool by Joen Wolform. Helps me a lot

I think that if she likes her finished quilt that should be all that matters to her quilting friends. Every one of us has different color schemes that we really like doing and someone else may think it is ghastly but whose quilt is it? I like working with brights and every class I have taken the teacher tries to talk me out of the colors I chose and I believe my quilts are perfect. I bet her log cabin quilt will be awesome!

I with you on that....for shame on those who receive a quilt made with love only to hide it

Sound like she has some good quilting skills, even some that you haven't mastered. How about a trade of skills - you help me with my FMQ, etc. and I help you with "new" color selections that I've learned. You're not telling her choices are lousy, rather you have some current choices to share with her.
On the other hand, has anyone, except you, complained about her color choices - maybe there's nothing wrong with her selections, theyre just not to your liking. I think I'd try the "trade a technique" approach if she really a good friend - otherwise she's appparently happy, leave her alone.
hmmmm......