How to check the security of your Google Chrome extensions

The other day, a friend called me and said that his Google Chrome browser is acting weirdly and may need reinstalling. According to him, every time he searched for something on Google some unknown website appeared with suspicious results. I asked him to remove every Chrome extension and only add those few that he really needs. But it occurred to me that this friend of mine will probably be hit by more scams brought by other malicious Chrome extensions unless he looks out for a few things. And I