I will try to make some suggestions which you may or may not find suitable:
Talk to her about it. She obviously understands something about politics, explain her more about it, be ready to answer her questions.
(If you are not a supporter of Trump, which we can't know) tell her that you personally do not wish him to become a president, promise her that you'll take her to the voting room (if legal/doable where you are) and that you'll vote against him. Explain her that this is what elections are about and this is how we speak our opinion.
Show your support of their friendship.
On the other hand, don't let your emotions to get involved in this, it would be very difficult to handle her fear if feared the same thing.