I need a good argument against veganism, can someone help?

She is absolutely correct. Unfortunately she is also being a complete bore by preaching about it, but she will get over that side of it once she's matured as a vegan. Sorry I can't say anything in defence of your so-called "normal" diet other than to say that the human body can cope with (and benefit from) SMALL quantities of animal flesh, not the three meaty meals a day that most wealthy westerners eat. However, it is the production methods of that meat that makes veg*ns choose the more humane option, as the human body can function just as well on a plant-only diet.