I gave my friend my password so he can Download some of my games. And he told me he downloaded a game because he just wanted to see if it would work. And he was able to download the game and put money in my wallet. How can I avoid this?
You say he put money in the wallet? So he charged your card? Did he charge for a game you hadn't bought, meaning he literally bought the game....or he downloaded a game you already bought and he put his credit card information in and gave you money? One thing I know people do is they put the credit card information in - put money in - then remove the CC info.
As said above you can also go to such lengths as changing the password after he has accessed your information as well....but some folks find that kind of a hassle. But as long as you aren't just handing the information out to complete strangers and you genuinely know the guy than you should be good. But when you give your information to a friend and he signs in, the credit card information is still there (assuming you didn't remove it) so he very well could add money to the account - that is if someone stoops that low....which is a little ridiculous.