If you have any pull what so ever I would try to get the owner to go with Executive Jet Management. That's good for everybody. I think they currently operate two SPXs and one straight Astra.
Otherwise, JetSelect out of Columbus, OH has an Astra SPX on their certificate. It's based in BCT. JetSelect has bases in SDL and VNY as well. However, JetSelect doesn't seem to know how to charter an Astra. They have about 9 Lear 60s and that is what they specialize in chartering.
Priester Aviation out of PWK has an Astra SPX that is based in FL. Priester has aircraft all over the country, but they seem to specialize in retail charter and not necessarily broker trips. So if the owner is looking for a lot of charter revenue, Priester might not be for him.
Reynolds Jet dba Air 10 out of LUK has an SPX or two. The CP is a good guy and Air 10 seems to be a good place to work. I don't know exactly how much flying they do however. I think enough, but they don't have any bases outside of LUK that I know of.
Wing Aviation out of HOU has a couple of SPXs. I know there are a couple of Wing guys on FI, so you might be able to get the low down from them. From what I understand Wing is a good company, but I don't think they have anything outside of Houston.
Air Charters, Inc. out of TEB has one. I don't know anything about them however.
Well, as you might be able to tell, I have a few contacts at some of those companies. Let me know if you need some help and might need another pilot. Whatever you do, if the owner wants to save some money don't base it with a company inside the VNY FSDO district.