I'm not sure here but I would say that there is no way you can make that happen from RoboHelp. I believe it will always open in the way you describe, according to the user's browser settings and operating system.
If there is a way to do it, my gut feel is that is something that your developers will have to implement.
As a software user, I think it would irritate me to find new browser instances opening but you have to go with what your client wants.
See www.grainge.org for RoboHelp and Authoring tips
Peter, many thanks for your response. I have discussed it with the client team, and a developer will be working on the solution.