The persecution relies in their own evidence of involvement. If it is actually true the other person wasn't involved, things will likely go well for them. If the prosecution has evidence that the other person really was involved, but someone is trying to cover it up, then the person trying to cover is likely in much more trouble, and the other person is more likely to be charged.
the co-defendant is but a single witness for/against the other. The prosecution may have other witnesses against your friend. And any good prosecutor will have a good theory for the judge or jury about why the co-defendant "tried to take the case." One defendants attempt to exonerate the co is a complex and tricky business. sometimes its a good idea to try to do this, sometimes not. but It can be done.
The admission of co-defendant is not a controlling event in a criminal case. It is just one of the many pieces to the puzzle. The preparation and presentation of the co-D's admission is the task of the criminal defense attorney. It may be weak or strong in protecting the accused. In some case it can "backfire" and be used as an indication of involvement by the person that it was meant to protect. Be Aware and beware.
This answer/response is based on the information provided in the question asked and requires a much more complete context than is available in this public forum. Please do NOT use this answer/response to say or do anything regarding your situation. BEFORE you say or do anything consult with an experienced Federal and/or state criminal defense attorney in your jurisdiction who will listen to you and your concerns.
Both need their own lawyer. The one admitting guilt has helped the other at his own expense. Whether the statement of guilt is enough to have the case against the other dismissed depends on what evidence the prosecution has to the contrary. Usually in federal court, the Assistant US Attorney has quite a bit of evidence suggesting that both defendants were involved and they rarely back down short of a trial and certainly not because one of the two is trying to take all of the blame onto himself.
Please do not confuse the casual and impersonal exchange in this or any legal chat room with entering into an attorney-client relationship. If your situation warrants formal legal assistance, you should begin the process of hiring Mr. Aarons at email@example.com or another lawyer to consider all of the relevant facts and law in your case.
Agree with all of the advice given, you both need independent lawyers asap. Two warnings: 1. With drug crimes it is possible for two people to be guilty of the same thing, you don't want a situation where someone admits something that does niether any good and actually harms the case 2. If you are close, boyfriend and gilfried, married, friendly or even if not particularly close and are working together to "come up with a story" (even if it's the truth), it seems to me you run the risk of getting charges for interfering with a witness or obstruction, perjry etc. especially if gov't ends up proving the story is not true. Both these points are yet more impetous for each of you to retain an attorney yesterday.