Its an excellent idea.

Now is this an international connection? I'm not sure if she would be able to
do a thru checkin, but i'm sure someone will be able to confirm this.

Easiest way is to take the NJTransit and I think its the fastest.

Once in New York, there are plenty of options. It all depends on what
she would be interested in.

Museums, Central park, Times Square, boat rides city tours..
Restaurants... etc..