I have coeliac disease and my DH is veggie. We tend to cook the same thing for the whole family. If your teenager friend is a coeliac then yes, cross-contamination will be an issue so use separate knives, clean chopping boards, separate pans, colanders, stirrers etc. As you can tell, it's just easier to make everyone gluten free and veggie for the weekend where possible IMHO.
We have a lot of veggie curries - chickpea or kidneybean or aubergine - in our house since curries just involve spices, onions, garlic, tomatoes and whatever you're currying.
You can also easily do a veggie pasta sauce (maybe with a carton of ready-cooked lentils for nutrition) because again pasta sauce is usually just herbs, tomatoes and fresh veg. Just buy some g/f pasta in the supermarket (which tastes much the same as "normal" pasta so noone should mind) - go to the "free from" (or similar) aisle and it'll all be there. Only thing to watch out for is that g/f pasta goes gloopy if you boil it for too long. But even if that happens, I just rinse it through and it gets rid of the gloop. Just don't overcook and it's fine. Oh and add a drop of olive oil if you're cooking spaghetti so it doesn't stick together, and keep stirring while cooking.
My DS has just woken up so will have to leave it there for now but check out the Coeliac Society website because there's probably lots more info there:
www.coeliac.org.uk/
PS while you're in the supermarket, you can stock up on g/f treats for her, like g/f bickies, hot cross buns (sains) and stuff like that. Oh and use the grill to toast anything for her since the toaster will have breadcrumbs in it. I just sling a piece of tin foil on the grillpan for quickness and ease to ensure it's clean, I mean, non-glutened.
DS has settled again, so, here's another thought while I'm on: if you're going into town with her, Caffe Nero not only does lovely coffee but also does a most delish g/f brownie thing. Hope there's a Caffe Nero in Cardiff.
Another thought for you is that I find indian takeaways are always a safe bet since indian cookery tends not to rely on glutenous starches. You'll have to ask them if they can cater for a gluten-free diet and be very sure they understand (many chefs still don't - they think gluten = wheat). I wouldn't take the risk if I were you since eating gluten for a coeliac can have nasty consequences and it might be best to play it safe for this visit. You can get more adventurous as time goes on perhaps?