I got stung quite badly myself a few years ago - shady character in Cork - well known for his poor quality cars. Subsequently, I will NEVER buy privately or from a small outfit again. Main dealer and def. memebers of SIMI. SIMI have a complaints procedure for members [broken link removed]
Your friend will have to stand his ground. Write a letter requesting a receipt. If he's as dodgy as this, I would question whether his tax affairs are in order. Advise him that you will be getting a Motor Assessor to look at the car. Perhaps offer him the opportunity to get his own independent assessment. When you have the report from the Assessor send it on by post and offer him the opportunity to either repair or replace the car as per the Sale of Goods & Supply of Services Act, 1980. Send both letters by registered post and offer him 10days to respond. If this fails, send him a solicitor's letter. You should get a result at this stage. The dealer will probably take his call during this time and your friend must insist that he will take this all the way. Advise that it's in the dealers best interest to resolve this ASAP because he will be hiring a car until the matter is resolved and the dealer will be liable for all costs incurred.