My colleague's first language is not English. He speaks English moderately well for someone who has been in North America for the last four years (I should know, I used to teach ESL). However, this guy is contacting many local businesses and even media outlets on behalf of our company. The signature on his emails reads, "Thank you for being my friend!"

My boss has seen this and not commented on it. I think it's HIGHLY unprofessional to use such a signature when dealing with North American companies. Do you think I should say something or keep my mouth shut?

Talk to the employee about it. Possibly its an english translation of closing used in business letters in the country which he comes from and he doesnt realize it is considered unprofessional here.

I think it gives a personal touch and if the boss has not said anything about it then it probably is not much of a concern.

If the boss hasn't said anything, I'd leave it alone. Really, is it HURTING you in some way??

Why don't you ask him why he signs his emails that way. Then you will know if he meant to or if he got mixed up with the english version of his salutation.

I immigrated from Scotland to the USA five years ago. The Scottish accent cuts me more slack than I would get if I had a local accent. That is human nature - people think I sound like Sean Connery. So maybe your co-worker has the same advantage. Perhaps the people who correspond with him find it "cute". Hey, if it brings in business or creates a good impression for the company overall, don't knock it. If, however, it does the reverse then you should tactfully draw this to your manager's attention.

Before you say anything you should ask him how long he's been using the signature for. If it's been a while then don't fix what' isn't broken. It's not bad to have a memorable signature, especially such a friendly one because it makes your organization easy to remember for the people who are in contact with him. IF it's new, you could suggest that he use a more professional signature, such as my favorite, "Best Regards" or for a longer one "Thank you for your time"

You should say something. What a great opportunity to help someone adapt to our culture and gain a valuable work-partner as well. Commend him on his desire to form business relationships and for understanding that for someone to do business with you, they must first like you and trust you. Then provide a more professional way in which he sign his emails, going beyond the "Sincerely".

Of course you should.