How much should I pay my SAHM-friend to watch my daughter?

I think you should ask your friend. But my humble opinion is that she should get 1/2 of full or nearly full pay for a nanny share hourly rate.

You should pay her no less than the minimum wage for the city in which the care will take place. Work is work, it doesn't matter that she is already home with her baby.
 

Agree that you should ask her. Since it isn't a huge chunk of time, it seems like you're paying babysitting rates. I'd offer her $20/hour or sitter swaps. Just because she's already home doesn't mean that it isn't a lot of work to watch a second. I remember the first few times I babysat our friend's kid (both boys were about that age) and it was EXHAUSTING. Much easier now that they're 3. 

You should pay her what you would pay a nanny in a nanny-share with one other child.

I think you should pay her the going- rate for child care or at least nanny share prices. She is a friend and working from home, why not support that work rather than denigrate it as SAH? Don’t take advantage! 

I think about $15.00 per hour is reasonable:)

Getting involved in financial affairs with friends is always tricky. If you want to keep your friendship on good solid ground, I’d recommend paying her more than you think you need to. 

I'm not sure how much the "friend" factor would come into play... it's still a huge job, especially at that age! I don't think I'd feel comfortable paying a friend any less than $15/hour, which is basically like a nanny-share rate. 

I agree with the posters who suggest you pay your friend at the going rate for care. I had a stay-at-home friend take care of my son--  her child, was younger than mine. She wanted to do it for free.  Financially, that would've really helped (my husband was a university student, I worked a rather menial job at the time.)  I am so glad I insisted on paying the going-rate.  Today, decades later, we remain friends.