What is the ruling on my friend receiving my prize on my behalf, and is she obligated to inform them that she is not the prize winner if I intend to give it to her as a gift?

There is no harm in your friend receiving the prize on your behalf, as this falls under permissible agency (wakala); due to a person's need for someone to act on their behalf in their tasks, and the Ummah has unanimously agreed upon its permissibility. It is not obligatory for her to inform them that she is not the prize winner unless they ask her, in which case she must be truthful with them. They have the right to verify that you have authorized her, and this can be documented in universities through the student affairs administration or other documentation bodies.