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