I think the best way to reduce her fear is to explain that what she fears cannot happen. You could read this excert from The Bill Of Rights together...
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof
Explain that her friend is free to be a Muslim in America, and there is nothing that Trump, or anyone else can do to prevent her.
America is a great nation, founded on principals of freedom, tolerance, and diversity. It has survived bad presidents in the past, and can do so again.
It would be interesting to learn where her fear has come from. I'm assuming if she's is frightened about this, then her friend is probably more so, which may mean her friends parents aren't aware of the limitations of presidential power in America.
The president is not free to implement whatever draconian laws he may want to. However there are certainly countries where a head of state does have this kind of power.
The best way for your daughter to overcome her fear might be to help her friend overcome hers.