[dave1000]

It's funny that Americans could tell your friend had studied in Australia by the accent when they can't seem to pick me as a (native) Australian speaker by mine! Maybe my accent is much closer than an English one from Dad's influence. My friends keep calling me a Pom (Australian slang for an English person) even though I was born and raised in Australia.

The previous comment about students having little control over the type of English they are taught is a good one, though with a bit of effort the student can learn remember the differences and stick with one form of the language. Word processors are a great help when writing - when writing for an American audience I just switch it to US English in the spell checker and write in my usual way, and it picks up all the words that I need to change!

When speaking or listening of course the spelling doesn't matter. Instead, it's little things like Americans not knowing what a fortnight is, or saying "off of" instead of "off" (eg. steam is coming off of it (US) vs. steam is coming off it (BrE)). Americans usually understand me of course. But if these little differences are mixed together by one speaker, it sounds strange (in my opinion). And if you are aiming to speak English well, surely you don't want to sound strange! So to me it's not a question of being understood, more a question of style and sounding "right". Whether that matters to you is up to you.