OP, here's your legally-sound (in the US, Canada and the UK) answer.
Since you already know she's disabled, you legally don't need a doctor's note from her to attest to this. If you'd feel more comfortable with a doctor's note, you may legally ask her for one, however, you're still legally required to recognize her disability as such while you await the arrival of said doctor's note. Regarding the accommodation issue, you are required to accommodate this employee's disability (even before she brings you a doctor's note), unless you can prove that such accommodation will impose a financial hardship (i.e., loss of income) on your firm.
Hope this helps!!!