How do I tell my friend that she needs to stop giving herself so much to do and that she's going to burnout?

You can say all you want to her, but you can't change a person's actions. This seems to be her natural inclination, and until she sees what it does to her and accepts it, she'll continue on. I think it's good to let her know you're worried for her- but in the end, don't work too hard trying to convince her- it will just stress you and your friendship out.