considering Mercon V is a different spec/fluid than ATF...and I've read warnings from Ford specifically about not putting ATF in this transmission (owners manual)...I would have to agree with the mechanic
read the back of the ATF bottle. does it specifically say it meets the Ford Mercon V specs?
all the oils on the market have a statment like this on them...and most list out the Ford and GM specs they meet. it'll say saying like "Meets or Exceeds the Specifications of Ford 000-a99, Ford 882-28s...etc."