I personally wouldn't have a problem with my 13yo going camping in this way, so long as you know the 40yr old very well and know that she will be supervised. But if you are stressing that much why can she not share a 'bedroom' with the friend?
It would be completely safe. DS, 10, sleeps in his own small tent. You can hear everything when camping. When have you EVER heard of any bad happenings on a campsite? Ony safety rules involve common sense - never ever light a flame in the tent.