There is no way to tell which module is the bad one unless there are visual signs of damage to it without installing it or having specialized equipment to the best of my knowledge. Honestly, I've never seen or heard of bad memory causing damage to a motherboard unless it was severely abused.
Did the memory cause damage to your friends motherboard that it was last installed in? If not, then I'd suspect it won't with yours either. Likely, the bad stick simply won't work. But I'd check with him on that to see if something else was damaged or not.