Dear Bryan;
It's not likely your friend caused a 'rear main' seal leak.
It could be comming from the side plate or pan gasket on the main cap, even the valve cover. He did the work, so It's his job to "make it right".
Good luck.
PS: Yes the (rope) seal can be changed with the engine in and the 'squek' could be a pushrod rubbing on something/rocker arm W/O oil etc.
