I played with my friend the other day, and I used hate ( http://deckbox.org/whi/Hate ) at the beginning of his kingdom phase.

"Action: Take 1 resource from each opponent and add it to your available resources."

The only way for him to use resources before I take one away, is to play a tactic card in response to my action?

He ment that he could play support and unites cards before the action resolved since he already were in kingdom stage…

Thankful for all help!

You are mostly correct. He can play a tactic in response, or trigger an action on a card in play that uses up resources, but he cannot play a unit, support or quest in response.

From the FAQ:

Playing a unit, support, development,
or quest card from hand are actions
with restrictions built into them. The
restrictions are:

They cannot be played in response to
anything.

They must be played during the owner’s
Capital Phase.