I'd be concerned, and I'd be looking at getting an EHCP. I'd question why they are entering her into the SAT papers if they aren't confident she'll get at least the lowest score.
However, I'm a secondary teacher, SEN specialist in a large mainstream school, our current year 7, when we tested reading in September, more than 10% of them got significantly below average scores - we are in a very middle class area and get excellent GCSE results, we normally get about 10 students with low scores and this academic year it was 36. We put intervention in place and now we only have 4 students with significant issues, which is where we normally are by summer term. When students have significant disruption, some of them really struggle, but when they are feeling secure, most of them make really good progress with the right intervention.
I would be wanting to know if this is a specific difficulty or a global delay - if it is specific then with the right intervention they may well go on to really well - I've seen several children get fantastic results at GCSE despite arriving at secondary with very low starting points. Got one in year 10 at the moment who couldn't read at all on arrival in year 7, now predicted 4-6 in 8 GCSEs. Had a student a few years ago go from predicted 2 to 7 in GCSE Eng Lit when we put a reader and scribe in place for her.
If it is global, and they are finding all learning difficult, then they need the right support in secondary school. We have very few children who aren't able to access GCSE at all and they can sometimes become quite isolated at KS4, not always, but sometimes an alternative placement is better.
And it is definitely possible to go on to have a good life without any grade 4 GCSE's! I've seen kids go on to some really lovely apprentice placements and thrive outside of school.