Some of your answer will depend on whether you mind them sleeping together. It’s only a question of your comfort level that they are, not whether or not they should be. I know my husband would never share a room with a young woman who is not his daughter. So I think the studio would be out. Also is the other kid a boy or girl? You could put your daughter in the second room, sharing a bed with the girlfriend. But if you have a son, I wouldn’t put two boys and a girl together. We have brought my daughters boyfriend with us but got a two bedroom. He had his own room and bath and she had the couch and shared a bath with us. We have also brought my daughters best friend and got a one bedroom so they share the living room and we have the bedroom with a shared bath and two doors into the bath so the kids don’t have to come through the bedroom to get to the bathroom. Also how will you share expenses? Will she need to buy her own airline ticket, park ticket and meals or will you pay? We always pay for friends that come but that’s our choice.
Will the young adults go out on their own at night? How will you set expectations for curfew at night or getting up in the morning? Will you care if they go clubbing or drinking if of age? What expectations does her parents have? I think there are more issues than just a room, but every family is different.