Again, it’s not good practice to turn on phantom power if a mic doesn’t need it, but short of an old ribbon mic (or a mic that specifically gives a warning), there will be no problems having it turned on even if your mic doesn’t need it. As for “never” using phantom power if the mic doesn’t need it – that’s a little extreme to me but if that’s what he was trained to do, that’s fine. However, in many cases, that’s not possible. For example, many low to mid level audio consoles switch on phantom power in groups or even to the whole console at once. In those cases, you will regularly have non-phantom powered mics plugged in that are still receiving phantom power. It won’t hurt anything.
As to whether there are different versions of the k6 power module, I don’t have an answer beyond the two versions you already noted (K6 and K6P). You would have to check with Sennheiser but I imagine your friend’s is one of those two and his misunderstanding of phantom power is what is causing your confusion.
Jeff
