What someone can't do now isn't always an indication of their future capability.
When I was 16, I got a bass guitar. I'd never played an instrument.
My best friend was a drummer. His uncle taught me Under My Thumb by the Rolling Stones.
It took me two weeks to learn it. I never learned another song. I never practiced. Never learned notes, chords, scales, music theory.
A month later I sold the bass at the swap meet. Shortest music career in history.
Or so I thought.
In college, I bought a harmonica. I practiced every day. I studied Sonny Boy Williamson, Little Walter, Rod Piazza. Within a year I was sitting in with blues bands.
Then I taught myself guitar. I practiced every day. I wrote songs. I recorded CDs. I played in cafes and restaurants and bars and house parties and festivals.
Twelve years ago I joined a reggae band. We played for six years. Then the bass player quit, so I said I'd be the bass player.
I practiced every day. I already knew the fretboard, notes, scales, theory. I learned to clip the notes, not start on the one, leave space, drive the rhythm.
Way back when, I wasn't capable of playing bass. Now I am.
People are capable. You just gotta believe in them and give them time to develop.