Tail Feathers

My friends Cockatiel seems to only have 1 tail feather left:



You can see it on the left hand side there sticking out. What he wants to know is, will they grow back at some stage?

'Cause they were ALL there and then suddenly they were gone, and left him with only one. He can't even fly (unless you help him by giving him a lift off) his wings hang down, and (sometimes) when he walks his wings will hang down & the poor little thing is so desperate to fly when he's over my place, and sees all the other 'tiels flying. Soo can anyone help him out here.

I feel really bad, not knowing what's going on, or why this is. I have not had any problems like this, as my 'tiels all have their tail feathers, and always have.