It would be strange for someone to do this particular degree without background in sport at a decent level.
That said, Exeter, along with Loughborough and Birmingham, are known as the top unis for Sport Science, so having a degree from Exeter would have an intrinsic value per se, due to the calibre of the institution.
Like others, I'd encourage her to consider a more "solid" option, given the academic ability she clearly has. My eldest did a degree in Sport and Exercise Science, but this was back in 2009 when fees were much lower; she was adamant that sport was the only thing that interested her (and was competing at national level in her events - international level with her club). Given the dearth of jobs for school leavers, and the fact that apprenticeships were not really an option back then, I encouraged her to do the course she wanted - if nothing else she would have a degree and have a good time growing up and standing on her own two feet.
She graduated and worked in the sports industry for a while - long hours and low pay. Now she is a primary teacher and enjoying it - coaching athletics in her spare time. One of her friends did a Masters and is now a sports scientist with the England Cricket team, but it took a few years of further study and unpaid internships for that to happen. A lot of graduates from this degree become PE teachers. She needs to think very carefully.
My younger daughter was very similar to yours in terms of GCSE grades; she read Physics at a Russell Group uni, and I was happy that she picked a robust option. She is now happily training as a Medical Physicist.
Plenty of time to consider her options, so it's good that you've raised this at this stage.