I had a lazy eye/squint/amblyopia, which was operated on when I was four.
You would now never know it ever wasn't straight to look at me, but the eye is still lazy, so consequently the good eye is getting progressively weaker.
I only have vague info from my mum, whose story has changed over the years.
Apart from the fact that I suspect she didn't have it investigated early enough (in spite of my auntie insisting I had a squint from early toddlerhood), she always said I stopped wearing the eye patch when I started school (me being five and all, she obviously left the decision to me!), which left the neural pathways undeveloped.
Then recently, when I was looking into whether I could 're-train' the eye, she insisted that I wore the patch for as long as the doctors specified, so I was just unlucky.
I'm afraid I can't say what normally happens nowadays - but I think it's positive for your friend that it's been picked up so early. There's no reason her DS shouldn't end up with a straight eye, and hopefully, if she can keep up the patching, good vision as well.
I'm Agra