need help now bov install gone wrong

Here is the scoop. I sold a 95 AWD turbo to a buddy(mistake). Well some dumb a** he knows installed a HKS blow off valve, venting to the atmosphere. He blocked the dump tube with a wad of electrical tape. Since the install the car has been running so rich, raw fuel literally pours out of the exhaust along with a huge plum of white smoke. Not to mention the check engine light is on. The car will idle around 900-1100rpms, and while driving it stumbles like a drunk old man. what can I do to rectify the situation??? PLEASE HELP