If he didn't put the o-ring in the proper place at the top of the threads of the filter carrier, a leak will result. There was a little paper sheet with the filter that shows the position the ring belongs in.

Note arrow "1", the position of the O-ring is critical; many beginners place it at the rim, in this photo, that means the ring is to the left near the rim and not in the channel provided.

Also shown is a collapsed paper filter (yellow arrow), which is why we do not use paper filters.