Hugely depends on how much your sister's friend will fly.
If flying for more than 100K miles, then AA is the clear choice out of NYC.
Complimentary domestic upgrades and 8 systemwide EVIPs are the main reason why I would consider them the best FF program out there.
UA has really limited service out of NYC, so unless one goes to Asia frequently, I'd stay away from UA.
CO has EWR hub, so it may or may not work for your sister's friend.
Living in Long Island, I find it terrible to go to EWR over JFK and LGA.
I can't comment on DL as I haven't flown them for a quite a while.