Is it legal to inform an employer of another employee's actions that can potentially hurt the company?

I currently work for a large software development company. One of my coworkers is a less than quality employee and is talking to other coworkers, including myself, about quitting his job next month with no notice. I'm sure he's serious because he's also mentioned a family member is getting married next month, he'll be attending, and he hasn't requested the time off. He mentioned just quitting and relocating to another state in time for the wedding. He plays a vital role in our development process and his leaving without notice would cause a ton of issues and setbacks on our current projects. If we don't have a replacement before he quits, the company will lose quite a bit of time and money. I want to warn management about what he has said about leaving the company without notice. My question is, would sending management an anonymous email stating exactly, "Employee's Name has mentioned during conversations that he will be leaving the company next month without giving notice. I am concerned about the potential consequences of his actions in regards to our current projects and clients. I wanted to inform you of this information so that you can either inquire further or find a replacement. Thank you, Anonymous" be legal? Since we are a development company, I'm sure if they really wanted to know who sent the message, they could. Is this legal and what are any potential consequences? I'm not worried so much about the company knowing it was me that sent the message, but more about this guy finding out it was me and claiming slander or something similar.