Car cranks over and shuts off

Car was running fine then my sister in law
borrowed for a few days and it stopped running
she put gas in it and it still wouldn't run, Now we've
changed the fuel filter,  still nothing. Bought a fuel
pump thinking that's it but haven't replaced it. I had
a friend from work check it out he says when you
turn the ignition to "on" he hears the fuel pump
running. He doesn't think it's the fuel pump
because fuel comes out of the end of the line
under the hood but doesnt come out with good
pressure.  He thinks maybe it's the fuel injectors
some aren't working and some are. My brother in
law says maybe it's the fuel pressure regulator
which is easier to fix. We bought starting fluid too
to double check opened air intake sprayed some in
cranked it still nothing. Any advice on what could
be the problem here?