UHLEEK, you are right to be concerned about your friend.
IIRC the test for whether your friend is legal is whether there are legal uses for the parts she has. She is still in the clear if she has other uses that may be illegal. The court will not presume that she will be acting in an illegal way instead of a perfectly legal way (unless, of course, there is other evidence that she intends to act in an illegal way).
For example, if she has a pre-ban gun that she could put the collapsing stock on, she is fine. This is the case even if she has post-ban guns that could also take the stock. Likewise, she could be worried about breakage and have some spare collapsing stocks for her pre-ban. (Obviously if she has 1 pre-ban gun, 50 post-ban guns and 50 spare collapsing stocks, there will be a LOT of explaining to do.)
On the other hand, if the only use for the part is an illegal one, she is in big trouble.
For example, if she only has a post-ban gun and she has a collapsible stock that will fit that gun (but will not fit any of her other guns), she has a problem.
The leading case in this area is, IIRC, Thompson Centerfire.
Your friend, you, or some of the other people on this board, may or may not feel that this restriction is one of the stupidest pieces of legislation to come down the pike and that it does not achieve its stated goals of reducing crime.
THAT DOES NOT MATTER. It is still the law and if your friend violates it she will be in BIG trouble. Do not take the counsel of those who suggest "Who would know?" There are too many ways that the police could find out and the costs are too high. (For example, what if she has to use the rifle to shoot a person threatening her in her home? The cops will check her gun too. Then she loses the chance to ever own or even touch a gun and may get an all-expenses paid vacation in her local prison.)
Tell her to either get a pre-ban gun or ditch the collapsible stock NOW.
The foregoing should not be considered legal advice. Your friend should check with a lawyer (and get one ready to call if she continues to act in a potentially illegal manner).