http://www.theguardian.com/technology/2014/apr/07/uk-governm...
They only have option b) if they won't to avoid this in the future. Open source or not.
A big example is Gov.UK which is mostly mostly open source as well as a few other big UK gov services like police.gov [1]
A large part of the NHS was moved off of a proprietary oracle system just a few months ago [2]
[1] http://github.com/alphagov [2] http://www.theregister.co.uk/2013/10/10/nhs_drops_oracle_for...
In any case, if they have the usual volume licensing deal with Microsoft, they can use any version of Windows without charge.
Sounds like a good startup opportunity.
They're also heavily regulated and if they can't show evidence that they've done all the required audit checks to ensure they're in line then the repercussions make millions look like chump change (think an investment bank being banned from trading on certain desks).
Depending on their priorities and at that size this might be the most economical move.