If a French-speaking person told you that they have the cafard, would you know what they meant?

In French, avoir le cafard means to feel down, blue or to have the blues. The word cafard [ka.faʁ] means depression, sadness, melancholoy. It also means a false devotee, hypocrite or bigot; an informant; or a cockroach [source].

It comes from the Arabic كَافِر‎ (kāfir – unbeliever, disbeliever; farmer; ungrateful), from كَفَرَ‎ (kafara – to disbelieve, cover, conceal) [source].

Related words and expressions include:

• un coup de cafard = a fit of the blues
• attraper le cafard = to get the blues
• donner le cafard = to depress
• J’ai toujours le cafard les lundis = I always feel blue on Mondays
• cafardeux (-euse) = glum, gloomy, depressing
• cafarder = to sneak, to sneak on, to tattle (on), to tell tales, to rat (sb out), to blab, to grass up, to dob in, to tittle-tattle
• cafardage = sneaking, talebearing, taletelling, tattling
• cafardeur (-euse) = snitch, squealer, tattletale, grass, telltale

How would you describe someone who informs on / betrays people, or a cafardeur/cafardeuse, and what they do (cafarder)?