We all know Martingale will not work in the long run, should I let him find out himself but that will incur a heavy loss in this case.
Or should I discourage him by showing him videos, articles, and testimonials?
Because of this, I will not talk about online gambling to newbie I could put a guy in big trouble.
I would not say stop him from what he is about to do but it would be perfect if you gave more insight about it and of course, explaining about the martingale's success is not much will help him to realize that strategy is nothing but a faster way to bust our bankroll.
And importantly, I would talk about the money he wants to deposit, I will insist him not deposit any money that he considers he can't afford to lose and if he is taking the risk then he also has to face the consequences of it.