David of Broadway wrote:

It most certainly is permitted to give or lend a MetroCard to a friend
while you're not using it - even an unlimited card. (Of course, that
only applies to standard-issue cards available to anyone - Student
MetroCards and Reduced Fare MetroCards and employee passes and the like
are not transferable.)

I should add that, in the case of a pay-per-ride MetroCard, up to four
people may ride on the same card at the same time. If they transfer,
they should only swipe/dip once the second time; the proper number of
transfers will register.
--
David of Broadway
New York, NY, USA