Sorry is the hardest thing to say

Your friend was being honest with you by saying something in the first place. Take it as a learning curve about your friend. Now that you do understand what had hurt your friend, it is time to simply tell your friend "I am sorry for hurting you. I didn't intend for it to hurt your. I am glad that you said something. It will be some thing that I will learn from. So thank you for being honest with me instead of keeping bottled up." That may help you. Also please remember that this is only advise that I am giving you. If you feel that this advise is not right, I understand and I do respect your decission.

You do not always have to say sorry by the five letter word. You could spologize by telling them you did not mean to hurt them and that you were just joking around and you really did not mean to hurt their feelings in any way. That helps in some situations.