ha ha.... what a load of BS.
It's a decent looking car, but the entry level 245PS TDI Allroad with a couple of interesting extras, then an S6 badge and some heavily spaced aftermarket wheels...
Even the "Sport" (S-Line) Allroad should have LED headlights, Sports seats and sills... and if he was going to do the fake S6 thing properly then he should have started with the S6 pedals, sills and steering wheel, but that's >£1000 of hardware so probably not worth the investment: hell even my old A4 TDI quattro Sport had an aftermarket S-Line body kit, genuine S4 alloys, R8 RBMFSW and RS4 pedals (and an S4 RARB), but there's no way I was even going to put an S-Line badge on it...
That said: if we give him credit for genuinely having the tech pack and if the aftermarket wheels are good quality, then the mileage and spec adjusted valuation opens up quite a lot: from £12,900 (poor) /£15,200 (good) for a non-adjusted private sale to £9,800-£18,200... I guess the wider range is due to the high-ish spec, but higher mileage, so it will depend on how well the car has worn those miles and how good the service history is (i.e. for 20,000/year, it probably ought to have 2x services a year!)... and for a 5 yr old car, they may or may not be Audi services.