Clearing wouldn't be available to her, but she could drop herself into Clearing (if she wanted to) by using the "Decline my place" link:
Obviously
before she did this she'd need to make sure that the place she wanted to "upgrade" to was available. Even then, declining a confirmed place is a gamble because there no guarantee that she'll then get the place she's hoping for via Clearing.
If she's actually in Clearing (which she won't be initially) she can call up uni and provide her Clearing number, they can then use that number to access her full UCAS application and make an "decision in principal" on the phone. She then has a short window to formally apply for that place via Clearing (perhaps 24 hours or so), after which the offer expires.
But, as I say, this is only once she's in Clearing. Prior to that (whilst she's still holding her confirmed place) she could call up some target universities and make informal enquiries, but the universities won't be able to see her full application (as they won't have her Clearing number), won't be able to make any "in principal" offer, and may not really be that keen on having the conversation as they really need to be talking to students who are actually in Clearing.
The other option, of course, is for her to decline her place and then apply for admission
next year with her actually (better) grades. But that means taking a gap year (which she may not want to do) and there's no guarantee of her getting the place she wants anyway.