@colditz
She may understand that eating quinoa instead of mince saves a cow, but does she understand that it starves Bolivian children?
I mean, no it really doesn't, at all. I'm not sure exactly what you're basing it on, but firstly, the majority of people in severe poverty across the world very rarely, if ever, can afford to eat meat and instead eat grains. The beef in South America is generally exported for us fatties to eat. I agree that there is a huge problem with distribution of food globally, but that's an entirely separate issue. Also, vegans don't actually eat more quinoa than any other vaguely adventurous person 
And that having palm oil margarine instead of butter avoids the dairy industry but it kills the orangutans?
Palm oil is an incredibly complex topic. It is actually a very efficient, and potentially very environmentally sound, product (far more so than all other oils), but the way it has been intensively produced in the last couple of decades is obviously awful. Some advocate total avoidance, and others advocate supporting sustainable methods. That's a whole other discussion, but regardless, it is in probably the majority of processed products in people's cupboards. Surely you don't believe that vegans are the only ones who should be responsible for this?
That the world is suffocating under plastic and acrilics instead of wool, silk and leather are not helping?
Plastics and acrylics are an issue, I agree, but I've never heard them blamed on vegans. How bizarre. There are plenty of plant-based fabrics that don't come from an animal- and again, does the responsibility for this fall only on vegans? I suspect almost everyone reading this thread owns polyester.
That if we don't eat the honey, nobody will keep the bees - and the plants she wishes to live on won't get pollenated?
Sorry but this is utterly false. Bee-keeping actually harms native populations- just like cows that are bred for milk are not the same as wild buffalo, so too the bees raised to farm honey are not the same as native bee populations. The native populations are at risk of being outcompeted by the 'friesians' of the bee world. Experts are still trying to work out the scale of the impact, so I won't claim to know all the details- but eating honey is certainly not saving the world, I'm afraid.
A lot of this comes down to people expecting an 'all-or-nothing' approach. I have never met a vegan who claims to be perfect or not impacting upon the environment in some way, but to say that veganism is ridiculous because there are other problems that humans cause is a bit stupid really. We should all do our best, and I think credit should go to those who try harder by sacrificing aspects of their lifestyle (meat, cheese, leather etc) for that.