These subjects are so tough in a world that claims to be tolerant of everything, yet is tolerant of very little.
It seems that we are to openly accept Hindus, Muslims, Islams, Buddhists, witches, warlocks, atheists, vegetarians, carnivores, and Bose owners, however as soon as someone mentions God or Jesus, that is unacceptable and inconsiderate of everyone who believes differently.
With that said, and as a Christian, you are engaged in a business relationship with this person. I don't go to work talking about my beliefs, however it was interesting to see a few coworkers at the new church that I have been attending lately. Who knew. While we have a belief in common, I treat them the same as anyone else.
I have such a diverse group of people that work for me from all sorts of races, religions, countries, and backgrounds. I treat them all the same. If they screw up, they hear the same regardless of what they believe. If they do a good job, I do the same.
However, if any one of them, regardless of the personal/professional relationships we have, did anything that was inappropriate, offensive, or just plain non-productive, they get talked to. It isn't easy because the fear of someone feeling discriminated against, but again, in this world of tolerance, some things need to stay outside of the work place.
Talk to him. You can encourage the person on a personal level since you have a background, but you also have a business relationship and it is your responsibility to tell him that people are offended. If he takes the approach of "It is my belief to share my faith." then you can say that you are obligated to look elsewhere if things can't remain 100% professional while on the job. There is nothing illegal against setting the expectation of working while being paid to work.
Good luck.
