Why does hot water only last a couple minutes?
Regarding the crossed pipe, I think that your friend meant the hot pipe is connected to the cold port (and cold to hot) on the water heater. As Madura commented above, this question sounds similar to, "Why is my hot water running out quickly regardless of other household water usage?"
Essentially, if the cold and hot are reversed at the tank, or if the dip tube has been lost, that could explain things. I'm not actually suggesting that this is your problem (it sounds like you know better than to connect the pipes backward); I just think that this could be what your friend meant.