What would you do in case you saw a friend using bribery to achieve some purpose?

Bribery in and of itself has no moral content. You can put simple exchanges of goods for currency in this category. I.E. Bribery isn't innately "right" or "wrong".

If you disagree with the result of the bribery your friend is expecting, then all you can do is build a convincing case against the use of bribery for whatever is expected of the bribe.