Well..
First off, I wouldn't do a doggone thing because I wouldn't put myself in your cousin's situation to begin with!!!
Next, seems like your cousin has some choices to make. Either cooperate with the Feds or risk going to prison.
Also, if she cooperates and become an informant, then she is at risk of getting harmed by the person she is trying to take down.
What is the best option? IMO, there is none! She is in a predicament!
If she can get less than a dime piece in prison, then she should take it! But, do that if she doesn't take the stand on her supplier that way she won't have a snitch jacket on her.
Me? I wouldn't tell. Don't wanna be a snitch because I know that they run paperwork in the Feds and I wanna be able to program in good standing on any yard I program on..
But, in your cousin's situation, I don't know how women run their politics behind bars nor do I know how much time she would be facing should she go to trial.
As B Brazy from Who's Who on YouTube channel would say, "Shouldn't have done it!!" *shrugs*