What if I do the opposite of a losing investor?

Let's say I get someone (friend/family) to invest and trade stocks. When they go long on a stock, I go short and vice versa. However I close the position when they do. If the vast majority of traders lose money over the long run, then surely this strategy would work?

I can't do the opposite of what I think since it wouldn't be my true emotions and I wouldn't have the same thinking. However if I were to do the opposite of someone else who doesn't know that I am then it would work.

It can't be that simple right? I know, that despite most traders losing money, it's over time and not on a single trade. I could account for this by just risking a smaller percentage of my portfolio on a single trade. My friend in this situation could also just get lucky and end up making gains, but over time it's bound to come down, right? I could also use multiple people and average it out. I'm not sure right now, just blurting out what's on my head.

Obviously I dont want to be exploitative and make them lose their money for my gain. And so I would provide a small amount of money to them to start trading with. Therefore they wouldn't lose anything due to this.