Can you tell us the manufacturer of the DVR and is he wishing to just view the cameras or does he need to use the control suite?
Just wanted to check as some DVR's view clients work with just a web plugin, however if he needs the entire control suite your best bet would be to use a program like Parallels desktop http://www.parallels.com/products/desktop/
Basically you would load the parallells desktop which will create a virtual machine that can be executed from an icon on the Mac's desktop.
Once Parallels is loaded you would load windows XP or some other Windows operating system into the Parallels program using its built in wizards so that when the customer wishes to use the Windows side of things all they do is double click the Parallels icon and the Windows virtual machine will boot up in a window on the desktop.
One other advantage of this sytem is that if the customer decides to archive any recordings to the Windows/Parallels desktop it will also be available to the Mac OS so that the files can be copied or emailed just like normal.
Let us know if you need more info or clarification.
Codeweavers.com has a program called "Crossover MAC". It sounds like a much simpler option than having to install an intire PC operating system as well.
That would be a great idea if it would work however if you look at the compatibility list for supported applications the Bosch Remote View software is not supported http://www.codeweavers.com/compatibility/browse/name/
What you have to bear in mind is that the software must access multiple ports depending on if you are using the DDNS feature on the DVR so the software has to not only run in an emulated environment but it has to integrate seamlessly with the Mac's networking hardware to communicate from either the inside or outside of the customers network.
The parallels desktop can seem to be a little involved, however it is really very easy to setup and use and would offer a well rounded solution to your client.
If the client does not wish to spend money on the project and is looking for a cheaper alternative then you may also wish to consider using the free program Boot camp which is provided by Apple. http://www.apple.com/macosx/what-is-macosx/compatibility.html
You may also wish to contact Codeweavers and see if they will be able to integrate the software package for you in which case Crossover Mac may be a viable option.