If Yahoo is a reseller for them, they won't do anything. I've tried that in the past and they tell you to consolidate the reseller company. Personally I'd stay away from places like yahoo and google etc.. There are many other companies that are out there. Godaddy, Namecheap, Namepros etc.. I can go on.

That's why I usually asked my clients about their domain, especially if their very new to hosting etc..

I hope things get resolved with your client. Let us know how it goes.