Your friend is correct (assuming that's a typo, and he's saying men should not use soap).
There's a common misconception among women that a vagina is its own special thing with its own rules that somehow don't apply to male anatomy. This is not correct- there is a lot of biological similarity between the penis and vagina. Just as soap will irritate your genitals and upset the bacterial flora causing thrush, soap will also irritate the mucous membranes of the penis and upset the bacterial flora there, also causing thrush.
It's common knowledge that the sensitive head of the penis should be washed using warm water only, and I'm not sure why you thought otherwise.