How effective is the implementation of the DNA ThetaHealing technique for removing viruses by conversing with them to negotiate their leaving the client?

This is very effective. And, the reason is the conversation is supported by Creator and that is how it can be effective. There is an interplay of consciousness involved using this approach, and it will make all the difference. If there is sufficient belief quotient, it will happen. Your client who had been suffering with herpes you healed, becoming symptomatic again, was because she was re-infected by her husband. So, this was the reason you felt it didn't work. The healing was effective in the first instance.