Your dick leaking would not be related to sounding unless you are sounding all the way up past your urethral sphincter into your bladder.
This is not a generally good idea unless you have the equipment to maintain sterile sounds, sterile lubricants, and are gloved as it can introduce bacteria into the bladder and give you one hell of an infection.
Another thing to keep in mind is to NOT use ordinary objects to sound... you want to invest is some nice surgical sounds of surgical steel that can be properly cleaned and have no nooks and crannies in which bacteria can hide.
The nice thing about prince's wand
is that they are usually drilled their entire length, enabling you to cum with the sound in place...
Tell your friend to not go too deep- no more than an inch or so longer than his erect cock. Use a smooth sound... and drink plenty of cranberry juice to keep the urethra clear.
Sounds also come in graded diameters... and over time, you can gradually work your way up to larger sounds... stretching out your urethra... but take it slow.