A. Fork over the $200, get her parents off her back, wreck her budget. Apparently her parents' thinking is that the $ gift should match what the happy couple are spending per person on the wedding - so if the happy couple are having an elaborate bash (we all know what we think of that, so please don't bother commenting), then DD's friend is up the creek.
B. Fork over what she can afford (which will be $100 or less, not sure, DD didn't go into detail) and have her parents and who knows who else in the family mad at her.
C. No other options, she is committed to going, has to do something.
A is actually pretty common thinking, although it actually flies in the face of all etiquette. No gift should be asked for (particularly cash!) and wedding registries these days tromp all over that ideal. That said, it's generally accepted by non-sticklers of etiquette that a registry is ok, and a gift expected (though one should still not ask for one). In fact, people who believe this sometimes actually think you should give a gift PLUS the cost of your plate. So if your plate was $200, you'd give say $300 then. (Note, the cost of a wedding plate is MUCH higher than you'd think most of the time.)
Often the gift amount will also increase based on on whether you are bringing a guest or coming alone.
I would suggest that the friend tell her parents whatever of the following she is comfortable with:
1) She was invited as a guest, she is an adult, the gift amount is between her and the couple and it is rude/crass etc to inquire - unless they plan to gift on her behalf?
2) She is living within her means, which includes a budget for events such as weddings. As she is sure her parents don't want her to go into debt, she will be giving a sum with which she is comfortable. (And if the couple cares more about the presents than her presence, she won't attend.)
3) That it's improper etiquette to request a specific gift, much less hint at cash.
If the family is one to count and remember later, I strongly suggest a personal gift that *can't* be bought in the store and tabulated. For my sister (as college student) I made a cross-stitch with their names, wedding dates, flower, etc. For two close friends - two lap quilts. You could do many things - a friend offered to make us delicious handmade caramels for wedding favors for us as a wedding present which we were thrilled to accept. Sometime people offer to DJ/take photos etc depending on the wedding.
Re amount, the standard is you should give based on what you can afford and your relationship with the couple. Here on this board I suspect you'll get a VERY different idea of what that should be than elsewhere, so make sure the friend understands that it's not mainstream necessarily. We're probably against the mustachian flow. (Most mustachians will probably give much lower sums. We believe gifts to others are important, and this is about what is done in our circle.) As a single, newly minted college grad, I gave $50. I've now increased that amount to $150 for weddings my husband and I attend together, more for weddings of close family or friends. For wedding #1 this summer (good friend) I am making a queen-sized quilt. For wedding #2, I am giving cash as requested by the couple, of $150. (It might have been larger to $200, as we have been discussing increasing the amount, but for the fact that they couple invited us to the wedding late, which required us to pay $200pp change flight fees on top of flight/hotel costs out to them... Some would argue I shouldn't count it though.) I dislike giving cash, but as they noted, they are living in a small place in Hawaii, and will possibly be moving from there (with crazy shipping costs) after the bride is done with schooling. For wedding #3 (DH best man, although they are not super close - former coworker friend), I used sales, double sales, coupons and specials to buy $465 worth of product for $160. Part of that "product" will go for the bridal shower gift too though.
Finally, she might also consider going in on a group gift.
So in sum: Don't feel obligated to give cash. Give what you can afford. If you'll be made uncomfortable by that, decline to do to the wedding. It's between you and the couple only. Give outside the box (check first that it'd be appreciated though) or pay close attention to deals, to keep the cost down. Go in on a group gift.