Just because the prior poor-performing employee missed a lot of time early-on doesn't mean this employee is going to be poor performer, too. I'm all for firing when you know you have a problem, but I didn't hear that here--unless there's more poor performance or lack of honesty.

Plus firing somebody when they just barely quit their previous employer leaves them without any income and in a really tough situation trying to explain why they're out of work with no income so quickly. It's a bad look for your company, too.

If my biggest asset was literally underwater, and you did that to me, I would sue, even if I were an at-will employee. She could bring a Title VII disparate treatment sex discrimination suit if she knows you let a man slide (who didn't have flooded house) under similar circumstances, plus try to allege a detrimental reliance claim and age lawsuit if she's over 40.