The short version is, this is a question for their CPA. That said, the IRS doesn't care a whole lot about allocations like this so long as they are consistent. Whatever they use today can't be changing every other month.
I could tell you stories of partners and division presidents nearly coming to blows over this very question. The bottom line is, there is no such thing as a ""fair"" or ""right"" way to do such an allocation. My thought is, they just have to pick one and go with it.
With the blessing of their CPA, of course, assuming that even matters in their case.