Using a lost sim card?

My friend recently found a old, may be a company phone since it pops up the companys name on it. Its been a pretty long time(couple of weeks) now, and they haven't canceled it, or called to claim it back. So he took out the sim card and put it in another phone and is now using it, would he get into trouble what so ever if they find out that he was using it? Or, even get tracked down because of it?