Hi m8, looks a lot like Thrips to me, they are tiny little bugs that draw sap from the underside of the leaf causing a silver shimmer effect on the topside of the leaf. Use a magnifying glass, they are tiny dots to our eyes but ugly fudges up close.
They won't go away they also move and colonise any other plants in the vicinity. You can get a bug farmer to send you out a natural predator who when applied will exhaust the food supply...Thrips. Then quietly die...or you could gass them, or squirt them with some neurotoxin.
I'm pretty certain that's your friend's issue.
Hope this helps gl with the grow