It all depends on the details. Are there other corroborating facts that show why prosecutors should believe that the items belonged to this female and not to the defendant? Did she have smoking paraphenalia on her person, a prior record for drugs, etc. Or is this a situation where someone is willing to take the rap because they know that they will only get a drug class as opposed to someone on felony probation. I had a recent case where someone stepped up and admitted to a crime at trial, but its just not that easy. There are 5th amendment issues and also you need to evaluate on how the jury will see the "confession."
There's no guarantee. The DA already thinks of your bf as a bad guy, because he's on firearm felony probation. The DA's office may not believe the notarized confession. This case may have to be tried. The solution might be to demand a speedy trial, then put the witness on the stand to confess. If she has a problem with that, it will damage her credibility in the eyes of the judge and the DA.
This is more complicated than it seems. First, the "notarized" confession is inadmissible in court. The female must testify at trial in person. If she says anything different than what is in her "confession" she might be subject to other criminal penalties beyond the possession. She has the right not to incriminate herself--that is the Fifth Amendment privilege referred to by the other answers to this question. She also has the right to her own separate counsel before she admits to anything. That is her Sixth Amendment right. If she is fully advised of her rights by her own separate lawyer and still wants to "confess", then it might be helpful to your boyfriend's case. But there are no guarantees. The prosecution might have information regarding this female that could show she is lying for your boyfriend, in which case, your boyfriend might be facing additional charges of suborning perjury. This is a complicated issue, and your boyfriend needs to discuss these issues fully with his lawyer.