I too have spent my life being a mug, too nice (or weak?) to say "no" to anyone, and at times have been exhausted and stressed, but it is me, the way I am, and now I'm retired all those people I put myself out for have eventually told me how grateful they were. So if that is your daughter's nature, so be it, though there is no harm in mentioning your concerns to her.
I think you have wisely decided against saying anything directly to the "friend", and possibly even indirectly. Give your daughter some ideas for refusing to help this woman out, and then leave her to it. Be there for her when you can be, but at the end of the day it is her own choice as to what kind of person she is. She will get her reward some day,if that is "only" knowing that she did her best for others. What a lovely person she is, be proud of her.