this is a non-starter.
I worked as an usher for many years, I worked many many concerts. There are ushers on the sides of every section, with radios. There are security guards walking around the show and just outside the seating areas.
Nobody is going to call 911 in an emergency on their cellphone...Have you tried talking on your cellphone during a show? Neither the caller or the 911 operator would *not* hear a word said. You would call 911..the operator would get on say something and you'd say "sorry, can't hear you...hold on while I leave my seat, walk down the stairs and to a more quiet place."
It would be foolish for someone to try and handle it themselves, if your friend was having a medical emergency you would kill them. There is medical staff standing which includes an ambulance. You spending 10 minutes trying to get through and talk to a 911 operator would simply kill your friend. In fact, I bet if you told the operator that you were at a concert they would tell you to immediately hang up and talk to security.