I make curtains, and for the most part, either use a #14 or a #16. Either would work well.

Pima is the type of cotton plant--it has nothing to do, really, with the thickness of the fabric. You can get sheets made of pima cotton, knit T-shirts made of pima cotton, etc.

It would surprise me to find a really tough fabric made of pima, because it's not the cheapest version of cotton--only a little less expensive than Egyptian cotton. So I wouldn't think you waste that on a cotton duck or something similarly hard.

And it has a long fiber, so makes a soft fabric with a rich-feeling "hand." Again, not something you'd waste on a duck or other stiff fabric that needs a #16 needle.

A pima-cotton T-shirt is thicker than another cotton T-shirt, but not in a way that means you need a honking huge needle; it just feels richer and more substantive. But I'd totally use a 9 or 10 on that fabric (I have a pima cotton tank top from Lands End). OK, maybe a 12 if there were sections that were several layers (like the bottom of a henley placket). But a 14 would be way too huge.

So I'd go by the drape of the fabric. Since you can't see it, can you get your friend to tell you how well it crumples, and whether she's got any other item of clothing, etc., that she can compare it to?

If I were in your shoes, I'd tell her to use a #12. That's nice and safe--not too huge of a hole, but also not likely to break in a thickness of fabric.

Thanks for the responses. My friend did say that it was hard to even place pins in this fabric. Also there are places where the fabric will be maybe 4 layers thick. I think that is the area where she will have the biggest issues.

That's the info that should influence your advice; great that she could give you that intel.

So then, a much tougher needle. And encourage her to test out needles on a scrap of fabric, and coach her how to tell if the needle is too big (makes a big hole that doesn't disappear easily) or too thin (hesitates before going through, appears to flex a little, etc.).

Then she'll build her own estimating skills and her own ability to get her answers for herself (a feeling of efficacy).

Because it really isn't that difficult, and the consequences of making a mistake aren't that horrendously huge. (Well, maybe you can damage something in the machine if you break a needle because it's too thin--I don't know. But I break needles all the time, and my machine still runs.)
So encourage her to be self-sufficient and to feel powerful.

(I personally tend to err on the thinner side and break needles. So maybe I don't know all the clues for when a needle is too thin. Don't listen to me!)

Talley Sue, I tend to use the trial and error method myself. I do try to be really careful with my machines and I do try it out using scrap materials. I agree with you about letting her learn. I find that figuring out things for yourself teaches you a lot. Although sometimes that might be a little "iffy". I started sewing in 1963 and believe me I have tried quit a bit of unusual projects. That has helped me learn over all these years. It's fun seeing other people become interested in learning. I want to infect EVERYONE with my passion for making things. Thanks for you input.

Like you, Liddybuff, I end up being a source for people learning to sew. (Right now that's my DD.)

I find I have to fight the temptation to just answer people, and to be the hero with my knowledge, etc. And to instead show them how to figure out for themselves, or even to just say, "Your logic and reasoning sound good to me; you can trust yourself."

Or sometimes to say, "The stakes aren't high--it's OK to try but be unsuccessful. Give it a shot! Nobody's going to die, your machine isn't going to blow up, you aren't going to have horrendous regrets."

So, sometimes it's powerful to encourage instead of teach; it's always more powerful to teach than to answer.
And I -always- have to discipline myself to not just "answer."

I'm lucky in that my mother was really good at that. I'd call and say, "What size needle?" And she'd say, "here's how you decide."
And then she'd say, "Well, what do you think? What's your idea, and why are you thinking that?" So I'd have to work out the logic on my own, but I was in the presence of an expert who would say, "Hmm, that might be iffy; I had this experience once that might indicate otherwise."

She made me nearly fearless--I'll sew almost anything, even if I've never done it before. And even if it comes out a little lumpy.