Can I buy an iPhone 6 for my girlfriend?

Obvious throwaway. I want to get my girlfriend an iPhone 6 for her birthday since her iPhone 4s is starting to wear out (button not working, charger not charging, etc). Her birthday is coming Dec 6. The problem is, I'm not sure how to go about buying it. She has virgin mobile on her phone since they require no contract and she actually doesn't have a social. Do I just buy her the phone by itself and she will be able to use it? Or should I get a contract with one of the providers and use my ss? I'm not too sure what's the best option. Some helpful advise is appreciated.

Your gonna have to fork out quite a bit and get an "unlocked" iphone 6, if you can find one of those, generally on ebay or amazon is your best bet, then you will need a virgin mobile sim card, then she will have to call and activate or transfer her stuff from her current phone to the new one. Now i'm not sure with the iphone 6 being so new that this is all possible and since virgin mobile is horrible in my area i'm not to certain if they actually have sim cards for unlocked phones or if they are cdma only, but IF you wanted to do this that would be the best way

As a gift you pay full retail for the phone, so all she needs to deal with is her carrier plan. So if she plans to stay with Virgin Mobile, you will want to buy her a phone that will be compatible with their network.

But before you buy the phone you might want to see which other networks will work for her, with no contract, if she brings a phone to them, bought and paid for.