I agree here. I don't think we are comparing apples to apples. I can go grab a porsche or other 50k+ ride and spank the Audi all day long. AWD is going to be really hard to beat in a straight line race. Then again, this is like you racing a VW golf. If his mentality is drive what you bring and not a drivers race, then you may not win. AWD vs RWD is pretty tough to win against, but hey, you never know. Just go for it and realize his car is going to depreciate a lot more than yours and in the end, is 1 second really worth another 30k in the 1/4 mile? Not to me brother.
If he wins, don't forget that its about enjoying the race together. If its about who can spend the most $ to get the fastest car, then thats not racing, that spending cash on an exotice and challenging lesser cars. No sport in that. The Audi S5 is an awesome car. If he wins, consider that you both have to have a second run but you must drive each others car? That migh help him see the light.