First you need to determine if the stool looks abmormal, he is a link to a Zupreem brochure that has pictures of what normal stools should look like,
http://www.zupreem.com/pdf/bird/how_to_feed.pdf

The stool can be brown there should be some firm parts(see the pictures in the brochure)

The brochure also shows abnormal droppings, if the droppings look like these you will need to take the bird to an avian vet. Talk to your friend and see if she already has a vet, you should also try and collect some of the droppings in a container to show the vet.

Birds hide illness very well. You might only notice small changes in behavior eating habits stools, but the bird could be quite sick. The fact that you noticed a change at all since you are only caring for the bird for a friend is great. You are obviously paying attention. Many bird owners who have had their birds for years miss things like this.

If you have any doubts at all after looking at the brochure online call and avian vet for advise. Make sure it is an avian vet, amny dog/cat vets do not have the expertise to treat parrots and also miss the suttle signs they show when ill.

Good luck and I hope the bird is okay. You are doing a great thing for your friend, all birds owners need someone like you. I am one of the lucky ones I also have a goos friens such as yourseld who watchs my birds, There is not a boarding facility in my area that I would trust with exotics, so if I could not get someone to watch them in my home I would never go away. I too will be having surgery at the end of september and it is much easier to heal knowing someone is taking care of your loved ones human or animal.