She needs a Schengen visa unless she is admissible for some other reason not mentioned in this thread. Most commonly those using a Vietnamese passport to travel to the Schengen countries need a Schengen visa to visit the Schengen countries.
Originally Posted by
Bigzamboni
I'm not sure if HEL is set up to allow Non-schengen to non-schengen transfers.
It is. JFK-HEL-Asia and back gets a fair amount of use by those who are citizens of China and India but live in the US on immigrant visas or LPR status.