I would come up with a contract persay. Something that you would use for ANY parent, not just your friend. Something that puts out expectations that you both will sign.
Expectations of course being things like pay rate, payment due date, what happens if she needs care later than such and such time (extra fees?... For example, she's done with work at 5, but doesn't pick up til 7 and you really want to get back to family life at 5:30). Anything that is in black and white can't come back and bite you. If she has a copy that she signed saying she provides formula, or diapers etc... She can't come back and say that she thought you would provide them. Not to say she would, but with my own experience with friends.. They will use you for what you're worth and more, but it usually comes with a price. For me, it was many evenings babysitting w/o getting paid.
If you treat it like a favor to your friend, it could very easily become just that. If you treat it like a business, you can keep morals from getting crossed. Not to say that even being professional is perfect. It's not.
I personally for my home daycare, I charge $75/week for the 3 days. My sick days and vacation days are free. The parent's still pay if I am available, but they don't bring their kids. Payment is due up front.
Also, if you are planning on doing this from your home... You're only legally allowed up to 3 children (including your own). Which doesn't sound like it's a problem at this time.
Good Luck!