When my friend builds PC's he lines the motherboard trays with elec. tape to prevent any shorts. Is this a good idea?

Well it wont hurt anything as electrical tape does not conduct electricity unless its high voltage. As to the static discharge from the tape material I wouldnt recomend putting it on or near the mother board. But if the tape is attached to the metal on the case or tray than it would be grounded. You really dont need to worry about shorts usally due to the space between the board and case unless you do something other then the normal mount.

applying tape is a complete waste of time. this is what motherboard standoffs are all about. The motherboard should never touch or be near the metal of the case. Problem I see here is as the case heats up, which is normal, the tape will loosen making a gummy mess. Not only that but all devices including the mainboard need to be grounded. If the tape prevents that then normal static discharge, which should equalize by going to the ground wire, can't happen and your friend has accelerated the death rate of the assembly.