What does she want to do in the long run?
The thing I found quite frustrating about "easy" subjects (I did media studies because I genuinely wanted to work in the media) was that a lot of other people had picked it because they thought it would be a "doss" and so messed around and generally took up a lot of the teacher's time meaning they didn't have as much energy to input into those of us who actually took it seriously. I got a D in Media, having been a strictly A-C GCSE student, and probably one of the more computer literate in the class as well. I got my results when I was working in an actual media company - so was clearly more than a D student to get hired!
The other thing I'd want to warn her of (though she may not listen) is that A Levels are a completely different ballgame to GCSEs. You might coast through GCSEs with a fairly low amount of effort, if you are bright and suited to the format, but A Levels will tax you and will require a lot more work to be put in.
Art is not "easy" - it requires a lot in terms of scrapbooking, prep and pre-prep work, and can be hard to get a good grade in. If she likes art and is good at it then it's a good option to pick, but she will need to be motivated and expect to put a lot of hours in - much more than an essay based subject.
Sociology is one of the options people often pick because it's seen as "easy". If she's an A*-C student then it probably isn't a good choice for her. Psychology or Philosophy would probably be better. This is just because if she's always been in top sets she'll probably find it frustrating that others in the class are messing around. Plus Sociology as an A Level is not that well respected when it comes to uni/job applications. If she wants to go on to a Sociology related course at uni, she doesn't need the Sociology A Level anyway. If she was at a slightly lower level academically then it might be a good stepping stone, but at her ability if she went on to do Sociology later she will find she's going over a lot of stuff again which she'd probably understand if coming across it for the first time at degree level anyway.
It's better to do 4 subjects in AS and then most people drop one when moving to A2, so they can better focus on the three they choose. Very often the one they imagine they will drop changes by the end of the year. I think this is a really important point! Also, if she does 4 then she will have room for at least one less serious option if she wants to do this.
It is an unusual decision to make at 16, but perhaps it would be worth her taking a year out to do full time work if she wants a break from academic stuff? This will also have the effect that she will see what menial work is like which might give her a kick up the bum to start investing more in her education! 
If she needs to justify this later on UCAS personal statement or a job application she could say something like that she wanted to gain some real life experience before deciding on her options so that she had a good idea of what she wanted to do.
Did my A levels in 2007 BTW so I don't imagine things have changed much :)