I, personally, wouldn't do it. I'm happy to have DS bring a friend along on day trips (including water parks, amusement parks, etc.) but I wouldn't do it for an extended vacation, for a few reasons:
1) Disney is family time for us, and I think it would change the dynamic a lot. I might actually enjoy having an even number for a change, but I also might get overly nostalgic for "just the three of us" stuff. I'm not sure. (We also usually spend a couple of days at Disney and a couple visiting relatives in Florida, so that might be awkward for the friend.)
2) the cost - even though I would have no problem with the kid's parents paying for airfare and park tickets, I might feel funny taking money for a lot of optional stuff (or making decisions about restaurants based on someone else's budget)
3) the chance that the friend would get sick or hurt - I would feel awful enough if I spent my week at Disney in the hotel room because my own kid got sick. If I was there with a random kid while the rest of my family was having fun in the parks, I'd do an Oscar-worthy job of being little nurse sunshine, but afterward, I know I'd regret taking the friend.
(DS
has brought my nephew on a 2-night trip close by, and none of that bothered me. But I think I felt differently because it was family, less expensive, and not as far away.)
The other big factor - she has 2 BFF's. I don't know which she'd choose. One is wealthier and more like to be able to afford it but parents are less likely to let her miss school - the other not as wealthy and maybe more likely to let her miss. Super confusing!
This would stop me in my tracks even if the above didn't! It could ruin the friendship with the one not picked.
One other thing. For me growing up as one of 3 kids, I loved when my parents allowed one of us to bring a friend to amusement parks. Whether it was my friend or someone else's, that meant we had an even number and everyone had a ride partner. It's a silly thing, I know and I might be strange but I always stressed about one of us having to ride alone, lol.
That was really mature and unselfish thinking for a little kid! I don't think it's silly or strange at all.