My DD5 is much younger than yours, however DD is a "helper" and we've been in a situation that is similar though not exactly the same.
DD has a classmate who has fixated on her. This girl often bullies DD and becomes angry and manipulative when DD doesn't agree to play with or participate in activities the girl dictates. This has led to tantruming and the girl screaming I don't like you, you are not my friend.
DD is extremely sensitive and is very hurt by these comments. She wants to appease this girl (who has other issues at play) and feels sympathetic towards this student because DD does not want to upset her. DD also wants to avoid being verbally abused.
I've explained to DD that it is not your responsibility to fix this problem even though you care about your friend. However, if DD has any insight or information about what is going on the best thing to do is to come to me or the teacher and tell us and that information can be used to help her friend. The best thing she can do for the friend at this point is leave her alone. Of course I did this at the appropriate level for a five year old.
In our case, DDs teachers have done a good job of separating DD and the girl during class time. I've also made them very aware of the guilt/conflicted feelings DD has. Do the teachers have any plans to keep your DD and the girl separated? Do they have any strategies in place in case the girl has a tantrum about your DD? Have they given your DD any strategies to "take care" of herself when these situations happen?
I get what you're going through because I have an empathetic helper as well. I keep emphasising that sometimes taking a step back and letting someone else step in to help is the best thing you can do for yourself and the other person.
Good Luck!
