What is the name of the antibiotic? I may not be familiar with it considering that you live in another country. Many antibiotics can result in organ damage if not accompanied by food and water. I believe that a half full crop should provide adequate buffering. Your vet may be thinking that because the regimen will only last for a short time, no appreciable organ damage is likely. I am not a vet, so I can only offer you my opinions. And one of those opinions is that if I were you, I would personally be concerned enough to never deliver such a medication on an empty crop. Considering that you don't have access to a certified avian vet, the best course for you and Robert will probably be to act on the side of caution as far as administering the antibiotic. It could be that the concentration of the particular substance is low enough to be safe even on an empty crop. But I personally wouldn't chance it.
I'm unsure whether you already have this hand feeding guideline from kaytee, so I'll post it:
It's from: http://www.kaytee.com/assets/021/42699.pdf