I'm sorry your DD is still feeling unwell, Classroomblues.
I'm a Guide Leader and I have been going away with Guiding for nearly 30 years. I am SCRUPULOUS about food hygiene, to the point of driving DH up the wall (he was brought up with a rather more cavalier attitude!).
At a camp, I would have a bowl of water for handwashing, separate to the dishwashing bowl. We would have a large jerry-can type of bottle of water next to it, together with a push-dispenser bottle of soap, a roll of paper towel and a rubbish bag. Water goes into the bowl, hands get washed with soap thoroughly and dried, water goes into the hedge every wash or couple of washes.
We always ask the girls to go to the loo in pairs at night, and to wake us up if they are unwell. We used to have a lat tent with a chemical loo for nights, but nobody really likes using it (or emptying it!) so I would only use that if we were some distance away from the loo block.
If someone was being sick or had diarrhoea during the day, we would ring parents. At night, we would move them to a separate first aid tent, closer to the Leaders, then ring parents in the morning.
The combination of going away, nerves/homesickness, change in food and routines, and lack of sleep can and sometimes does cause a slightly upset stomach (e.g. stomach ache). In this case, we would provide TLC, plenty to drink, distraction and monitor the situation discreetly and carefully. Obviously if a child is really ill we would contact parents.
It is also not unknown for girls to suddenly develop a tummy ache when asked to do washing up or packing up at the end of camp!
On the last day we would contact parents if a girls was clearly unwell, but if she was slightly off-colour she will be going home in a few hours anyway.
Was there a parents' meeting before the camp? We don't always have a full-blown meeting, but do encourage the parents to contact us beforehand if they have any queries. We plan the whole thing very much as a joint effort between the Leaders and the girls, including activities and menu. We stress the importance of sleep, although this does not always get taken on board (!!), and of letting us know about problems. We do preparation in the meetings leading up to going away (although sometimes it is not obvious as we don't want those not going to feel like they are missing out). This is all standard Girlguiding practice.
Before you contact the leader, I would ask your daughter why she did not wake one of the leaders when she felt unwell. Did she have diarrhoea, and did she explicitly tell the leaders this? It is a bit embarrassing, but we cannot help if we do not know!