A good tomato sauce with a lot of vegetables is a basic. I do a sort of cheats ratatouille which forms the basis for a lot of other dishes, eg with rice, pasta or cous cous. Ready made felafel from the supermarket go well, they come in lots of different flavours, or fried halloumi.
Sweat down sliced onions, lots of garlic. Add in veg like chopped courgette , diced aubergine, fry for a few minutes then add a tin or two of chopped tomatoes and a good squidgy of tomato purée and cook slowly in a covered pan for at least thirty to thirty five minutes to let the flavours develop. This will make more than you need for one meal so portion freeze so there is always something available for the vegetarian to cook quickly.
Another good standby is to find a daal recipe you like, I use the coconut daal from the BBC website, find it under Geroge Alaghai, I add in more garlic and a bit of garam masala. Serve with rice or naan, steamed broccoli, yogurt, mango chutney. Freezes well, again use portion sized bags, the IKEA ones are good so the vegetarian always has a quick standby.
Ready made puff pastry is another standby for a quick meal, make an open tart with prettily arranged finely sliced vegetables arranged on the top of some tomato pureee, or fresh tomato, or home made sauce.
Learn some good main course salad recipes using substantial carb bases like cous cous, ready made tinned lentils, cooked rice or fancy pasta. Then add on things like roasted butternut squash or cauliflower florets,steamed or raw broccoli, small roast cherry tomatoes, small roast courgettes or carrots,roast beetroot, toasted almonds, sunflower seeds, sliced caramelised red onions, etc etc with a really good flavoured dressing. PS Not everything at once!! Chose one or two or three. Make a good quantity to last several days so it is worth while getting everything prepped and ready.
Look up red dragon pie , no dragons are harmed. Makes a lot so good for a famiIy meal. I think having some recipes that can be eaten and enjoyed by everyone while also providing stand by meals for one person is a good way to go if there is only one vegetarian in the family.