If flight duty (from sign-in to on-blocks at airport C) exceeds your flight duty limits - then it's illegal, and they should stand down at airport B or operate heavy crew.
No company that I know of imposes any restrictions on duty time (apart from e.g. 50 hours a week, 2000 hours a year etc.) - so if you can make it on-blocks at airport C within your flight duty limits, it should be OK.
Of course they'll be knackered when the make it back to A, but they can look forward to a 26 hour rest period, so could be worse. Oh, and if your lot have 50 hours a week, they've just used more than 50% of that weeks allowance, so some N/As might be heading their way
