Possible your friend has deleted the post.
If not, you can search your friend's all posts. Go to his Timeline and scroll down little bit, you will see three tabs on left upper corner: FriendName, Timeline, Recent.
Click Recent and select year, the page will auto scroll back and a fourth tab will appear: All Post.
Click All Post and select month, after the auto scroll look for the post you are looking.
You can search for the same from your Timeline also.
Go to your profile, click on View Activity Log, on the left hand side click on Comment, select year from the right hand side. It will auto scroll and show all the comments by you month wise starting from December.