bronnie98
OP very generously agreed to let him lodge...
Maybe so, but there's still a business element to the agreement, since the student is paying rent.
... on basis that she cooked.
Nothing to suggest the OP can't still cook, or that the lodger has demanded to make his own food.
It's clearly selfish because he could have waited until he moved out - it's a short term arrangement after all
No it's not 'clearly selfish'. There's no evidence so far of 'selfish' demands or 'selfish' behaviour. Deciding to become a vegetarian, whilst living in somebody else's house, is not intrinsically selfish. In many cases of vegetarianism, there's an element of altruism. Given that the arrangement is short term, that's less time for the OP to accommodate her lodger's diet.
...unless he's planning to pick out bits of meat from his dinner
Feels like we've gone back a few decades in terms of the perceived difficulties of accommodating vegetarianism. At the risk of stating the obvious, even meat eaters don't (have to) eat meat for every meal and meat can be, and often is, cooked separate to other elements of a meal.
Out of interest, if a child of yours wanted to become vegetarian, would you tell them to wait until they'd left home because of the inconvenience it would cause you?