Attempting to make another person jealous by boasting is a sin. Why? Because it is a tactic to make another person sin.
Jealousy is what caused the devil to want to overthrow God. When anyone is trying to cause jealousy in someone else they are trying to make that person sin while elevating themselves.
Attempts to make others jealous or to cause others to sin is strong evidence that the showoff feels insecure and lacking good sense.
2 Timothy 3:1-5
"But understand this, that in the last days there will come times of difficulty. For people will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, heartless, unappeasable, slanderous, without self-control, brutal, not loving good, treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God, having the appearance of godliness, but denying its power. Avoid such people."
When we see that someone is boasting creating a reaction, this is the time to begin avoiding them. If we are walking in the Spirit we will not be jealous and we don't want anyone to manipulate us into jealousy. It is true that bad company corrupts good character. The longer we ignore what they are doing the more likely we will be manipulated by them.
Be wise, we must make our companions those who love to please God.