Her probation should be extended. We have this girl in my workplace, she has been on probation as well and she was signed off sick for I would say 2-3 months now, not sure what is going on but honestly she has been off sick more than she actually was in work. Her probation has been extended now. In terms of your situation, upon employee return you should organise a HR meeting, it would be good if she had a note from the doctor or else explaining why she was off sick for so long. She should be explained as well how does her absence affect the work place in general, not to make her feel bad, just so she understands the importance of the job.
Not sure if you use Bradford Factor at your workplace but I think it does help managing absences. I hope it helps. :)

I'm sure somebody else will say it, but a probationary period has no meaning in employment law.

Address the sickness as you would any other. Consider occupational health reports and then look at your capability procedure going forward.

In reply to Klaudia:

Hallo and welcome here Maria!

This employee has already run up in not much more than three months since starting a sickness absence of about twice the national average for a whole year, so extrapolating the three months to a full year this is seven or eight times ‘normal’. All this should be put to the employee at a face to face meeting. As regards the probationary period, how is this worded? - for example does it expire by default in March or does it need specific confirmation that it’s been successfully completed?

In the circumstances, personally would wait a little while to see if they return bit before the end of March and if they do meet with them when they return and either if contractually permitted and appropriate extend their probationary period or dismiss them as unsuitable by reason of their very poor / unjustified sickness absence record and any other performance related reasons. If it’s not looking like they’ll return before the end of March, then you’ll need to do it by letter or email, but if you do decide to dismiss whilst they’re still away, best to offer a phone conversation or home or whatever meeting first in order to lead up to doing that and to listen to anything they have to say.

In reply to Klaudia:

Many thanks for your help Klaudia, much appreciated

In reply to Sam:

Thank you Sam - much appreciated

In reply to David:

Sound advice David, thank you - much appreciated

In reply to Maria:

You’re most welcome, Maria - hope it goes as well as it can.