No. Basically - if you can proove that the backtest information was wrong, then you can due. This is false advertising. If I say my system went long USD to EUR last year at parity (1:1) and the exchange rate was never close... then I make a false statement. Regardless what I may write into a disclaimer, this is fraud. Point.
That said, remember about comission, slippage etc. - often they are not included, and comission in particular can make a hugh difference. Plus make sure you did not make an error....
Check multiple times. Go through their trades manually and compare to the prices you have. Make sure your data is right etc.