You must play as usual. Any deviation from the normal hand-play due to the fact that you are playing with your friend is a violation of the rules of the poker room.
I'm not sure it's that black and white.
'Collusion' is what's not allowed, and collusion requires 'cooperation' between two players. If I make an unplanned decision to throw away my good cards, without knowing what my 'friend' actually has, how is that any different from me throwing away my good cards because I'm afraid of the shark at the table, who is not my friend?
In both cases I'm allowing my emotions to affect my play, but you wouldn't consider the latter example to be improper would you? 'Weak' maybe, but not improper. Most of us allow our emotions to affect our play from time to time.
It would be different if my friend and I agreed in advance that I would throw away my good cards whenever we are seated together and he/she raises. I think we'd all agree that's collusion, and that is black and white.
Besides, what is 'normal play' anyway? Who's to say what goes on in the minds of poker players?