Originally Posted by
GrumpyDiver
I think the answer is one can't know for sure. Lenses and cameras are exposed to contaminates all the time, including mould spores. This is how your friend's lens developed mould on the interior optical elements in the first place.
In the case of fungus, the risk is more related to how you store your own equipment as fungus needs to be in a warm, humid environment to grow. If you store your equipment in a humidity controlled environment, even when there is a high concentration of spores, the mould won't be able to grow.
The other issue is the location of the mould in the lens you plan to borrow. If it is in a part of the lens where there is no air movement, the likelihood of spreading the spores is lower than if it is located on the surface of a lens element that is subjected to air movement during zoom operations.