Exactly Marynary, Shockers yankeecandle4
The thing is, I guess, we need OP to come back and tell us more about what sort of person the friend is, and whether she is normally a stalkerish limpet needy type or a cheeky user, or whether she is just an as far as the OP knows normal person whose DC are good friends with the OP's DC...
I agree it is thoughtless and a bit odd to book in this way btw but I absolutely do not get why so many people hear "it will be great for the DCs to have friends on holiday." as "it will be great to have free babysitting" - the two things are not the same. We go on the kind of holidays where the kids make friends and run about the campsite with them a lot of the time - I am more than happy to be the one keeping an ear out for the kids if that's the price to pay so that my kids have some mates to play with, we never go anywhere while the kids play with other kids - we sit on the terrace reading and chatting and drinking wine watching the youngest and get up and check on him if he goes out of sight, and are the fixed point for the older 2 to check in with at set intervals or if they need help, and call them on mobiles or go and check on them if we feel the urge (and would if they were late back to check in with us, but they never are as they are used to a bit of freedom reliant on staying with boundaries).
We don't take friends with us but the kids make friends on holiday, and the older ones now keep in touch with holiday friends by email and snail mail as penfriends for a while When one family we met on holiday wanted to book the same week as us next time because their only child and our youngest got on so well and, you know, "its great for DC to have friends on holiday" esp if they have no siblings, we said sure, it'd be nice for the kids if we bumped into you again, but didn't exchange contact details - there was no expectation we would; we'll see if they're there in June! As parents we don't have much to do with the kids' friends' parents beyond a bit of chatting while watching the younger kids or at the pool if the older ones are playing together there - we don't sit together, it's chatting in passing. Sometimes we or the other parents will offer a coffee if we're near one or other caravan.
The ragey "kids must be with parents at all times on holiday even if kids would have a better holiday if allowed to play with other kids sometimes" does seem very British, and is probably why, with one very notable exception, our kids are always running with the German, Dutch and Italian kids on holiday (luckily they speak German, and with the Dutch and Italian kids DC2 speaks Football :o )