Northern European cuisine is about the worst for vegetarians, IMO, as it's so based around the notion of meat and two veg. If I'm fitting into that situation (like if everyone else wants steak) my core replacements are omelette, halloumi or veg pie/quiche.
If everyone is eating burgers/sausages, I'll eat veggie burgers/sausages. I wouldn't normally eat them of my own accord. The only other quorn product I'm really into is the mince, which I'll make in spaghetti bolognese (my secret to flavour is porcini paste but I'd bet marmite is also delicious). If someone else is cooking I'll eat any quorn/other substitute product they've provided for me.
The thing that works better is to look at cuisines which are less focused on the meat and two veg model:
Italian - pasta with all and any vegetables, pizza (preferably home made and thin crust), risotto, gorgeous fresh salads, gnocchi (tossed in oil in the summer, baked with tomatoes in the winter), lots of cannellini and other beans, in salads or as a meat "replacement" in e.g. lasagne or stews, etc.
Indian - anything revolving around chickpeas, lentils, potatoes, spinach, kale and other dark green leaves, cauliflower etc.
Thai - lots of varieties of stir fry (tofu is yummiest in stir fry IMO), pad thai, steamed vegetables etc.
Middle eastern - cous cous with basically anything, fresh bread with lots of yoghurty herby dips, baked aubergines, again lots of delicious salads.
I eat all of the above regularly and am a terrible cook so guarantee they can all be done easily! I'm more of a Jamie/Hugh cook than an Ottolenghi but you can be as adventurous as time/money allows with vegetarian food.
Just a few more thoughts:
Please don't feed her meat again. If you don't want to/can't provide her with a meal that fits her requirements then whether or not she's being reasonable you need to be honest with her.
Portion size is all. I don't think (from observation) that meat eaters are always great at judging how carbs map onto proteins and sometimes feed enormous carb/vegetable portions because it feels different to eating protein. It takes a bit of time to realise what full feels like when you're not eating lots of meat.
My diet is carb-heavy compared to meat eaters I know. It's also fat heavy because cheese is my indulgence (very little love for puddings). The carbs influence what type of exercise I focus on (lots of cardio) and the fat influences the amount (more than if I were vegan, for example). To my mind the whole thing comes as a package and no one should make big dietary changes without considering what impact this will have on their exercise if they want to maintain the same body (and I'm not making ANY judgements about her body, just saying that it all needs to be considered).