oh you are in a whole world of hurt...
1. Not often do you hear about these template sites being used here... I could only begin to guess why... they are incredibly hard to use. They're a great source for what do to on layouts and look and feel but that's where you should leave them. Personally. (not that were above template sites they're just not easy to use, I feel.)
2. you have someone who what's to manage their own site... I say Bravo! the CMS you'll use will depend on the size, subject and type of site it is.
3. does the site where the template come from have that template style available in the format or program your thinking of using? If so you could ask for exchange.
SO I'm going to agree a CMS is going to be easier for him/her to manage on their own. Joomla is a good choice, simply because there is a HUGE community of templates ready to configure to your needs. You could find something similar base colors and then toss your custom graphics from the other template and you should be good to go!