but that doesn't mean they can replace here whilst she's still signed off sick, does it?
Of course they can. All they need to do is ensure that upon her return, she returns to a job commensurate with her job prior to going off sick. Employers are allowed to redeploy staff, subject to a range of constraints which aren't relevant here. If she found the job she returned to not to her taste, it would be up to her and her advisors to see if it constituted constructive dismissal.
In a large employer, there is a constant turnover of staff. It's entirely reasonable to appoint someone permanent to replace someone off long-term sick, on the assumption that by the time they return another role appropriate for them will open up.
Are you seriously saying that if a nurse in a hospital goes off sick for six months, the hospital cannot recruit any permanent nurses until they return? Or that once recruited, a new permanent nurse cannot work on the ward previously covered by the person who is off long-term sick? Of course not. So long as there's a nursing job, at grade and specialism and hours, equivalent to that they went off sick from, that's all the returner can expect.
You are employed at a grade, against a job description and set of terms and conditions. So long as those are maintained upon your return, that's the end of it: it's the same job. You can't say "oh, I worked on the second floor with Ann, now it's the third floor with Betty, that's not legal." Because it is.