How to tell someone we don't want to live with them? (medical reasons)

Although this may seem harsh, we do feel that having one of the girls who currently lives with us live with us again next year would not be good for anyone. The girl is lovely and we all get on really well, this year and year previous she has been quite ill in and out of hospital. Firstly, her medical state and being so prone to infection would indicate she isn't healthy enough to live in a cold student house. But also the amount of work / lectures she has missed means she will probably have to retake the year for the third time.

We are worried that it is highly likely that she will drop out leaving us with the financial burden of having to pay for the extra room something which none of us can afford.

As this is our second going into third year and it would be likely to be her third time in first year we will be gone at the end of the year leaving her with no one.

How do we tell her we are not comfortable putting a deposit down for a 6 bedroom house instead of a 5? We need the guarantee that she will be here but health being something out of her control she cant promise she will. We don't want to be harsh or hurt her feelings but can't see how we can tell her we cant live with her .