I was just about to reply with something similar to what Phil said.

A few of my friends are Mormon. Lovely Christiam peoples who would do anything for me, however they call it missionary work and try and I guess their heart is in the right place (from my experience as I draw a line in the sand)

Personally I firmed up in my mind what was and wasn't acceptable.
Allowing them to say a prayer in front of me is acceptable, unless they are trying to guilt-trip me into doing something I do not wish to do.
Me praying is not acceptable.

They don't like foul language , innuendos and the like. Again that is ok with me.

If anything I would write down how you feel and what you feel is acceptable to you. Be honest with him about how far you will go with it. Then just keep checking back to the feelings and acceptability at the start and make sure he doesn't push boundaries...