Here's some facts that might reassure your daughter:
Herpes virus (HSV) comes in 2 varieties. One variety prefers the lips, causing cold sores; the other prefers the genitals, causing genital herpes. Both are infectious, and it's possible to be carrying the virus and not have any signs of it yourself.
Although the different types of HSV have different "areas of preference", that is not cast in stone. It's perfectly possible to get an attack of genital herpes caused by the type that usually causes cold sores. If that happens, the attack is often less severe and a bit less likely to recur.
It's not difficult to spread herpes by oral sex, if the other person has a cold sore (even if it's not very obvious), or you can "autoinoculate" on your fingers if you have a cold sore yourself - or, of course, your partner can spread it on their fingers if they touch a cold sore then your genitals.
So it doesn't automatically mean that either of them has been playing away. Nonetheless they should both go for a full STI screen when they get home, of course.
Meanwhile, has she been given any antivirals (eg aciclovir)? Bathe regularly and gently in salt water, don't use soap etc, and if she can take ibuprofen, that can help.
Disclaimer -- used to work in sexual health.