Stand-alone Child Theme

I am a marketing guy, not a coder. I have installed perhaps a dozen WordPress sites for clients, installed a free or premium theme they like for them, and turned them loose. I have a new client that wants me to rebuild their site for them after a bad experience with an unscrupulous web designer. Short version, they bought a domain they like, found this guy, and had him build a custom theme, supposedly do SEO, and blog occasionally for their small business. In short order, they came first in a search for their business in their town, which is all they want.

But this guy has done nothing on the site for a year, just auto-billing their credit card $1000 a month for hosting, and their ranking has fallen. They tried to end their relationship gracefully, but the designer has locked them out of their own admin area, and is demanding a “termination fee” of over $2000 to “turn over the keys.” Their site is hosted on his servers, and now a Whois search for their domain shows him as the owner.

They could take him to court, and they may, but that may be costly and time-consuming, and they want to get back up and running with a new domain in the interim. I used SiteSucker to download their entire website, and it is on my desktop, fully functional in html. However, I can’t get it into their new domain, getting a message that “the template is missing”. Googling around seems to tell me that it is a child theme based an a theme called “Protheme 3.1”, which I cannot find anywhere. I have their style.css file, but apparently not the parent theme.

How should I proceed? Can I make their custom child theme a stand -alone theme, or do I need to start from scratch?