To achieve this you'll need the site admin to be a paid partner. http://helpx.adobe.com/business-catalyst/partner-portal/transferring-sites-partners.html
Only then can he raise a ticket and let us know that he agrees to the site transfer and our team can cover the process. Here are the partner plans, so the client is aware of the costs involved: http://www.businesscatalyst.com/partners
If this is not an option, the only scenario where there would be a workaround available is that where you only have this particular site created with your partner portal, in which case you could just make the admin(the client) a partner portal user(raise a ticket for this). From that point on, for any new sites you'll have to setup a new partner account(basically you're hading over your partner account to the client).
Thanks for that great response, Alex.
Two follow-up questions.
1. Would they need the premium or standard partner package?
2. Would the site fall under their "Free BC site" instance, thus eradicating their need to pay ongoing hosting fees? (This would offset the cost of paying a partner membership and would thererfore be desirable).
Standard will suffice (any paid plan). The site will be transfered under exactly the same pricing plan, so if it's webcommerce now, it will stay as webcommerce then.