Friend charging me rent from the blue?

If it was agreed as you say, that you were not responsible for rent, then technically, you owe him nothing. It is wrong for him to first tell you that, only to spring a bill on you after the fact. It would be gracious of you to offer some assistance, but since you were not expecting to have to pay, you might have planned your finances with that in mind. Also, anyone who pays rent has a say in how the household gets run, like inviting whoever you want, coming and going as you please, Were you given the same rights as an official tenant? Anyway, he was wrong and its good of you to pay it just to save the friendship. Lesson learned: Come to a specific agreement in advance or get your own place.