The thing is that she is suing on the basis that the hospital haven't told her where the infection came from.
They may not know. Knowing where an infection doesn't make a lot of difference to its treatment, unless it is a highly contagious infection where protective measures need to be instituted urgently. Even so, it's very difficult to track these things down. There's nothing to suggest anyone else in the building had the same infection at the same time.
Even if someone else did have an infection with the same species of bacteria, labs don't usually waste time and money determining the 'strain' unless there is overwhelming need to (e.g. antibiotic resistant). Without strain information, it would be very difficult to prove that one patient infected someone else, or not.
What if they do investigate and they find that she brought the infection in herself? A lot of fuss is rightly made about hospital-acquired infections, but a signficant number of these are brought in by patients. This is particulalry the case when someone falls ill shortly after arriving in hospital - there hasn't been time to pick up infections from elsewhere, but the bacteria that had previously been living on your skin, could easily enter through an open wound. (Because of this some surgical units in hospitals now test patients when they book their scheduled surgery - and if they test positive for potentially infective bacteria, they are given a course of treatment, and only given an appointment once they are all clear).