belt slip on 99 taurus

OKay, posting here for a friend, i don't touch fords whatsoever, but here goes. It's a 99 Taurus, when it rains the belt slips of the idler pulley and hence goes to no use. Happened twice now, only happens when driving mild speed through 2 inch puddle. I took a look under the hood and the only thing i can see wrong is the idler has the slightest, and i mean slightest it took me 10 minutes of close scrutiny to find that the idler seems to barely be tilted. Any ideas why the belt would only fall after going through water and wouldn't be falling off any other time if the idler was tilted? any possible solutions and price estimate would also be helpful

is this car missing its splash shield or something?

I would first make sure that all the grooved pullys line up with each other ( one doesn't stick out farther than the others). They should be right in line with each other. If they all line up fine, I would suspect the idler pully needs replacing. If it is loose it can tilt under pressure and slide the belt off. If that does not work, then i would replace the tensioner arm and pully. Other than that, there should be no other way for the belt to slip off because all the other pullys are grooved to keep the belt in one place. As for price Idler $15, tensioner pully $10, tensioner arm itself about $60.

Does the tensioner give you an indication that the belt might be getting stretched?

If you have not replaced the belt yet you might as well do that if your are replacing the idler.

If you think it is a wet belt issue then try the theory out with a garden hose or bucket of water and see where the belt falls off in the driveway while idling.

thanx yall i'll pass the word on to him and see what we can come up with.