Please could you advise on what process we should follow when an employee returns to work after a spell of absence from depression?

The lady has been off work for just over 3 months and due to return tomorrow, however she has kept in touch by text and email on an ad hoc basis and tells me she will be back to work on 26th May instead. So there is a period when she isn't signed off by her doctor but not at work. I've tried numerous times to contact her, however she doesn't seem to ever reply. Finally she dropped by the office today when I was out and has said she'll come in to see me tomorrow at 11am to talk about things.

I'm concerned about bringing her back into work and how she'll cope. Should I get her doctor to sign her off as being fit for work? Or should we do an independant medical? 

We have a very small team of 4 employees so when someone is absent it is a real problem and the cost for the temp cover is expensive. Prior to this absent she had been off quite a few times with the odd day or week absence.

We are happy to try and support her, however I'm concerned that coming back to her job right now might make her ill again? She works on the reception desk. What should we do?