There are some seriously messed up people on this planet. The guy has nothing to gain by secretly feeding her dairy, nor does he have anything to lose if she refrains. It's all some sick action that has no explainable reasoning behind it.
It reminds me of a time years ago in the workplace, where a few people thought they would be oh-so-clever and pull a hilarious 'trick' on a Muslim colleague (who I also considered a good friend outside of work) by misleading him as to the ingredients of pepperoni (stating it was beef, and not pork). This ended up happening time and time again, until I found out (after noticing my friend eating a slice pepperoni pizza one lunchtime) and WENT BALLISTIC at this disgusting, disrespectful and childish plot. Again, these people had nothing to gain by him eating pork, and nothing to lose by him refraining.
Is it any different though, to a Vegan slipping meatless product into a meat eaters meal? I would say yes, it is different. Nobody commits to a 'all meat' diet, and they still eat fruit and vegetables. There is no compromising of a person's life choices and philosophy by doing so, but the same cannot be said for the reverse.