Even theoretically, if we take into account the likelihood of a series of lost bets in a row, as well as winning ones in a row, the Martingale strategy still has a limit on the size of the bet and therefore there can be no guarantee here. 
Moreover, this strategy is so well-known and widespread all over the world and known to all players that its value as some kind of interesting and unique strategy generally tends to zero.  So I think that your friend is simply mistaken in saying that this strategy is so good.   


And ask him a simple question  has he ever won really a lot of money using only the Martingale strategy.  Or at some point you still had to retreat from it, when the amounts become so psychologically significant that common sense prevails over the desire to continue the strategy.