few questions to answer.
Was car wating oil before?
How was the pick yesterday when you jump start it?
Was any smoke from silencer prior to this problem.?
Whats the milage?
I think your mechanic has put a reasonable guess, and yes cars can loose compression realtively suddenly. Avoid too many diagnostics, and if answer of above questions are in faour of weak engine, then you have the answer of your problem.