In my experience, I can operate pretty much any lock with my left hand. It's the clip placement or thumbstud that is usually the problem. Any Benchmade with an axis lock is ambidextrous. Also most Spyderco's with backlocks are lefty friendly. A Mini Grip, Delica, Native, or some of the knives in the Byrd line should work. If he doesn't mind using a right hand liner lock one of the Buck Vantage models or a Spyderco Tenacious are good values. There are some dedicated left hand knives out there but they are more than you want to spend.
Lately I've switched over to traditional knives for edc. They are completely ambidextrous. I've found that I personally don't miss the one hand opening or the clips, but you would know better what type of knife he's looking for. If you think he might like that type of knife you could look into a Buck 110 or a sodbuster. They are both simple knives with a classic style.
I have to say that he has a good friend. Not only are you going to get him a knife, you're also considerate enough to look for something that's easy for him to use as a lefty. Most people don't even stop to think about how hard it is for left handed people to use things that were designed for righties.:thumbup: