What if a Lib Dem minister votes for the boundary changes?

An interesting hypothetical question thrown up as a consequence of a piece from Isabel Hardman on the Spectator blog which mentions a "senior Lib Dem MP" who is quoted as saying that the Lib Dems should support the boundary changes.

Now I don't know for sure if this MP is in the government and it is not clear that even if they are they will not fall into line with their party but what if they decided to vote against the Lib Dem party line on this? Would they technically be in breach of anything? With the vast majority of government ministers (Conservatives) likely voting for the changes, wouldn't they just be another minister voting with most of their colleagues?

As we're in unchartered territory I'm not sure what the answer would be.

Other than Nick Clegg would doubtless be very pissed off!