I'm sure they are looking forward to the day and will be happy to help out in such a small way.
That's such a good point. People keep asking me what they can bring, and I'm like...*I* am making dinner! *you* are the guest! But what I really say is "Please don't feel obligated to bring anything but yourself!" If they insist, I have said "Well, you really don't have to bring ANYTHING, because I'm going to be doing all of the standard stuff, but if there's some special dish and thanksgiving wouldn't be complete without it, you could bring that."
If this friend asks me if she can bring anything, I might (keeping the "out" in mind) ask if she could bring some plates...
One other thought--I've been researching how to organize the hosting of T'giving, and I ran across a food guy's post in which he said, "accept help," and suggested steering people to those things that don't need the stove. And which might be annoying/stressful for you to fit in.
Like, salad. Or dessert.
Re: loaning plates. I'd absolutely ask and loan.
BUT...I would be borrowing plates a few days before, and I'd be returning them a few days after.
I would never schedule the borrowing/returning of dinnerware during the dinner itself. That's just too disruptive. And if you're hurrying, that's when things get broken. So don't tell her you'll absolutely clean them and replace before she leaves. Wash them, and pack them up for transport, later. If I were *loaning* you the plates, I'd be annoyed at having to deal with them on the day. It's not like I don't know where you live, and you might keep my plates forever if I don't get them back right away!!
Since you say these dishes aren't the "fine china" category, and that replacements can easily be purchased, if it were me, I'd ask to borrow.
I'd arrange the borrow/return in advance, but I probably wouldn't say, "what if one breaks?" because I'd assume that of course I'd replace it! Either with a good-condition plate from my own stash, or with a new one from the store.
Re: marking dishes.
Try out a china marker, and see if it stays on through the dishwasher or handwashing. If that doesn't, try out a drop of nail polish on the underside, beneath the maker's name. And that's smart--marking yours, not her.