I took my dad to a concert. He is not in a wheelchair, but uses a walker and really cannot manage stairs or walk very far. And he does have a decal for parking and a wallet card, ie, he does officially have a disability. When I booked on-line at Ticketmaster, I saw that you had to call a special number for "Accessible seating." The venue where this concert was held had a couple of areas reserved for people with disabilites and their companions. This meant that it was close to the doors, no stairs, and by an aisle, with plenty of room for wheelchairs etc. When we arrived there were staff there specifically assisting disabled patrons and their companions, telling us where we could enter, pointing out the seats, making sure we were comfortable, and making sure walkers, canes and so on were looked after in an area off to the side.
If your venue is listed as "handicapped accessible" then they should be able to provide a seat for your friend, otherwise it is not handicapped accessible. And handicapped doesn't necessarily mean wheelchair. Surely the person you spoke to was wrong.