Confidential informant

So I'm just needing some opinions on being a Confidential informant...my fiancé got pulled over and had weed on him...the cops let him go and want him to snitch on his connect! He has a good job and has now sworn to not touch it again...if he snitches he walks with nothing and if he doesn't he's facing a felony and will lose his $20/hr job!! Anyone know anybody who has been in informant? What does it entail...will anyone know he did it?