OK, that helps. But I have to ask:
1.) How does Company B make money?
2.) Why would Company A enter into such an arrangement with a separate company in an arm's length transaction?
Let me explain:
For a payroll outsource company, Company B makes money by controlling its expenses. Its prices include an allocated amount it puts toward overhead (and profit, of course, but that's not an important distinction for this discussion). But its actual overhead may be different. So Company B's profits come from the spread between actual costs and allocated charges collected. Likewise, Company A has a reason to be in the transaction: it saves money, in theory, by paying less in charges from Company B than it would incur if it did its own payroll. So the transaction makes sense to me; Company A saves money, and Company B bears a risk that can yield profits.
For a PEO, Company B makes money also by controlling its expenses. It similarly includes allocated amounts in its prices that it puts toward its own overhead. But again, its actual overhead may be different. So Company B is bearing a risk that can yield profits. And again, Company A saves money. It avoids HR costs from recruiting and retaining good employees and weeding out bad employees. So this transaction also makes sense to me.
What your client is proposing...doesn't make sense to me. From what you wrote, Company B will charge its actual overhead to Company A, along with all the payroll costs involved. So how is Company B going to make any money? There's no risk transfer that drives profits. Unless its transfer prices include a profit charge, Company B wouldn't make any money, and profit charges without bearing risk is more than a little squiffy.
And how does Company A save any money? If anything, Company A is going to have to pay for Company B's compliance costs--preparing its statements and taxes, legal costs, etc. These are all costs Company A can avoid if it just keeps in house everything Company B would be doing for it. So why would Company A enter into this kind of arrangement? It also sounds like Company B isn't really a separate economic entity, so I'd worry that GAAP rules would make you consolidate their statements anyway, and tax rules would flag the whole thing on suspicion of being a tax shelter or money laundering arrangement.
Maybe there are facts you've not told that make it make economic sense. Maybe your client is trying to gain a tax advantage or something by making its employees look like they're employed somewhere else. Maybe there's some other kind of sharp practice going on. I can't tell from what you've written. But from what you've said, there's no apparent reason why an arm's length transaction like this would exist between two independent companies.
If this isn't reasonable as an arm's length transaction, then that could mess you up with the IRS (IRC 267/267A and IRC 482 come to mind). And if these two companies are a single economic entity, GAAP rules may require you to combine their statements for reporting purposes anyway. So there's possibly significant downside here without any obvious upside. Maybe I'm missing it, but I don't see it in what you wrote.
So I'd ask your client some hard questions about the purpose behind this structure, if only so you don't get hemmed up for it.
Sorry for the long-winded answer. But your description makes me think it's not allowed or it's a bad idea for some good legal reason.