I carried out a little research on this recently for another thread. Without restating the whole thing, it does appear that UK based airlines have published policies on their websites indicating they honour the greater allowance on an interline ticket, generally airlines in other parts of the world operate on a leg by leg basis, even within the one alliance.

In this case it looks like your wife should be charged for any excess above the LH Business allowance of 30kg but below the NZ Business allowance of 64kg (I don't think any solo traveller could manage more than this amount!) across 2 bags. The charge should be collected for the LH leg between FRA and LHR only.