I went up Ben Nevis last summer, regularly climb mountains in the UK.
Fitness wise you will be fine- they might get a shock at how hard it is walking continuously uphill for 4-5 hours, scrambling etc. particularly the teen, but you will probably make it.
The bit that’s concerning is the lack of mountain experience. If you go up and back on a clear sunny day and start early enough in the morning, maybe.
But this is Scotland, the weather changes super fast: do they know what to do if fog comes down and they can’t work out which direction to walk in? Can they use bearings on a map and compass. Nevis has a huge drop on one side and multiple people have died simply by walking off the top of the mountain in the wrong direction.
will they be taking warm clothes and suitable boots/equipment? There is snow about a certain height on Nevis, all year round. Your boots need to be broken in and confident you will not get blisters as this will severely slow you down.
Have they practiced judgement about knowing when to turn back if the weather turns/they are slower than normal/injury etc? Nevis is not a mountain I would want to be walking down in the dark so you need to have a decent judgement on how long it will take you to walk back down what you have just walked up. Can be slower depending on how technical the terrain is. The top of Nevis has many switchbacks on gravelly, slidey slate and it’s a bit of a slog to get both down and up as you really have to concentrate
Take LOTS of suitable food, water, glucose tablets, snacks, you will burn much more energy than normal. Take more than you need for a day in case you get stuck/have to stop. Take a more layers than you think, Scottish weather is super unpredictable- we started in blazing sunshine and met fog and snow on the way up (in June). The weather forecast for the local area will not account for what happens at the top of the mountains, though I believe local mountain rescue in the area provide some more detailed info!
overall please remember that mountain rescue teams are volunteers who donate their own time to rescuing people from these locations and raise their own funds to do so.