Hello, my friend borrowed from me 4 digits money
I've lost in the past quite some money and "friends" by borrowing them money. I've learned on the hard way that this is a no-go.
I'd advise you no longer borrow money to friends. Getting loans back can be a dirty business, especially when sentiments are involved.
And my advise stands even if you get your money back and interest. Both investing and trading are risky and it seems that the only one bearing the risks is you.
does he obviously hiding a lot of earnings from me in day trading, that he would do from Mars to December
This is something only he knows.