I think it depends on why you are vegan. I think in your case it would seem ok.

Good question.

Really it all depends on how "devout" you wish to be.

Would eating the food cause you trauma?
Would it seriously invalidate your beliefs regarding the vegen ethos?
What makes you feel happier?

Its late and I am tired, so cant recall how vegan you are but I cannot see any calamitus wrong that will happen if you consume a small amount of whey powder..Unless you have made a firm stance at the extreme end of the vegan camp.
But then if you had doen that, then you would probably be living in a small hut in the forest of Dean, eating moss and lichens and wearing clothes made from dirt :P

The annotations on the first link are a bit biased.

For example in the first paragraph: blah blah blah have contributed to the options analysis =/= i.e. this document, or the content of it, is known by senior officials in ALL of these agencies.

Contributing to doesn't equate to full knowledge, and if you sit down with the high ups of Corus they'd be hard pressed to say what I've contributed to.

Interesting document though, always interested to see whats being considered, have a read on some of the environmental legislation thats being discussed currently a lot of it is absolutely barking!

I'd go witt the option that doesn't cause your mum huge amounts of stress. You're applying vegan principles to the rest of your diet which is a hell of a lot better than a lot of people can manage!

Mind Control!!!