A regular Google search gives about 350,000 hits for ‘indifferent between’, while a Google books search gives about 120,000 hits.
I cannot find any dictionary entries that include examples or citations with this usage, but it does appear to be in actual use, if not all that common.
As such, I wouldn’t classify it as ‘truly ungrammatical’, but rather perhaps as ‘best avoided in contexts where you wish to steer clear of potentially controversial grammar’.
In your particular phrase, I would suggest:
X is indifferent to whether (we do/he does/etc.) Y or Z
Or, even more plainly spoken:
X does not care whether (we do/he does/etc.) Y or Z
Edit to reflect edited question:
With the context now known, I would suggest recasting the sentence as follows (vel sim):
The risk adjustment should reflect the compensation that an insurer requires for bearing this uncertainty and reflects the point at which the insurance company considers fulfilling an insurance contract with a range of possible outcomes and fulfilling a liability with fixed cash flows (to be) equal options.
I share your disagreement with your colleague. largely has two common meanings:
- Mostly (¨for the most part¨)
- On a large scale
The first meaning is entirely inappropriate, because it is talking about the distribution of activity across the available options.
Largely it contributed to the understanding of Y but it also, at times, turned up new insights into problem Z.
So this is about how much of X´s application contributed to Y and how much it did other things.
The other meaning of largely also does not help, because while it does say that X contributed on a large scale, it does not say how that compares to any other input that may help with Y. For all we know, everything related to Y may be done on a large scale; it may simply be a big task however you approach it. It would be much better to phrase this in a way that clearly shows the importance of the contribution:
... technique X contributed very significantly to the understanding of Y
... technique X was the principal contributor to the understanding of Y
... technique X has been crucial to improving the understanding of Y
Any of those would emphasis the key contribution made by X and remove the risk of ambiguity. They are also more idiomatic.
That said, largely would work if the subject and object in the phrase were inverted:
... the understanding of Y was largely improved by technique X.
With it this way round, the mostly meaning of largely becomes appropriate, because it shows that X is the principal contributor.
Best Answer
Fulfill might work: