Without being cynical I'd have to say, from what you describe, that your kohai's problem is not your problem. To clarify that statement, you say his/her technique is good, the fear is a mental thing, so IMO all you can do is set a good example for your kohai technically, and when dealing with your own fear.
There was a good thread on fear in the Spiritual section about six weeks ago that would give some differing perspectives.
With the hope that it may help your kohai I highly recommend a book by a man named Jiddu Krishnamurti called Fear. Off topic for aikido, very on topic for fear! His teachings definitely gave me a better perspective on what exactly fear is.