That one sounds a bit dubious.
I've done a bit of Reiki myself as well as received it and afaik it just does not work the way as indicated in the story. Though what happened may be a side effect of the person's training in Reiki, similar to how some of Aikido's training tends to form someone into a person who tends to face conflict with a bit of benevolence instead of meeting fire with fire always.
Reiki means Universal Energy and it's designed such that the channel who is doing it does not actually extend his own personal ki, but acts merely as a conduit for the Universal Ki to operate on its own. All the person does is act as a guide for the energy and "aim" and "focus" it so to speak using a variety of methods.
Sounds to me like he is confusing extending positive thoughts or unconditional love (Ki if you want to call it that) towards the potential aggressor and this may have had an effect on the demeanour of the other person. It's hard to attack someone when they are interacting with you without any sense of malice but only projecting good thoughts towards you. Though this is by no means an infallible rule.
Reminds me of that Terry Dobson with the drunk in the subway story a bit.
This phenomena is dealt with in detail in "The Secrets of Shambhala" written by James Redfield. My Qigong and Reiki teachers have also indicated that when we extend pure thoughts of unconditional love towards someone their negativity cannot survive and it is transformed. Maybe this has something to do with the philosophy of Aikido and Reiki to some point, but I can't say that it is an application of Reiki as it is taught to many, at least imho.
Just some thoughts.