However bad you feel about this, I imagine this guy feels worse.
He must be a decent dude or you wouldn't have been "work friends" with him for the past 3 years.
He's away from his native country where all his close friends and family are.
He considers you a friend because you talk every day for the past 3 years and have hung out after work a few times. Consider yourself lucky that you have so many friends that you don't even consider this significant. But to him, it is significant.
I'm sure it was awkward for him to ask. He knows you don't consider him a close friend, but he has to get someone or else he'll look like a loser in front of his bride's friends and family.
I'm sure you're going to get some good food and free booze out of the deal, plus the chance to bang bridesmaids.
That being said.... if it were me I would've told him to fuck off.