Should / Would you let a mate know that he / she is just a fling?

So I just got off the phone with my friend who recently relocated temporarily to Nigeria for business and while we were politicking (guy talk) he informed me that he is now prancing around in a Range Rover jeep and that the owner of the car is a girl he is dating / sleeping with.  That will be fine except for the fact that the dude is married here in the states.  What really caught my attention is when he said that the girl told him to please not hurt her and he said to me that he was thinking to himself that "abi she should not hurt herself" LMAO.  We laughed for a second and then after I realized that this is not funny as the poor girl has probably started telling friends and family that she has a new guy who just relocated from USA.

So my question is should one just keep it real and tell a new acquaintance that this here is just a fling and nothing long term will come from it?  Keep in mind that if he had kept it real, he probably won't be getting free dinner, sex and of course riding around in the range rover.