Yes, you will owe her. If she paid up to 19th July and you had two weeks deposit from them, then when she leave on 26th, she only had one week fees attributed to notice period. You will owe her one week fees back.

It never come a good time but she followed your policies so she owed that money on final day of care, not later. Your financial situation not her issue just like if she had been laid off, it would not be your issue if she was struggling to pay fees.

I take two week deposit too for notice period but I require 4 weeks notice for this very reason.