I'm not aware of any biennial iris, your neighbor is confused I think. Certainly Beverly Sills should be perennial & should flower every year for you, but iris do need to be divided every few years.

They should bloom every year just like they are now.
I agree, dig and divide every few years.
Beverly Sills has also been reported to rebloom in places.
A thread to read.
Is Beverly Sills a rebloomer?

Irises are perennial, though they may not bloom every year if the growing conditions aren't optimal for them. That might be what caused your neighbor to believe that they are biennial. For instance, most TBs don't bloom every year for me because my growing season is just too short for them to mature the new increases to blooming size in a single season. OTOH, MDBs and SDBs (the dwarf bearded irises) do bloom every year here because they mature to blooming size much more quickly.

In zone 5 if you are providing your irises with full sun, Bev (TB) should be able to mature and bloom every year.

Laurie

I bought a beverly sills awhile back, I think at least 4 years ago. The first year it bloomed, it looked like pink insulation to me, ha ha! I had bought a black iris to plant next to it, so it would have some freaky color schemes. Guess what? Now it never blooms! My other ones bloom fine, but not her. >:( I even fertilize them with a little algoflash too...but nada.