From the information you’ve provided, I’m not sure I agree.
Whether Mr A wants Miss C to be a sub tenant is not the important point here. (And from the way I read it, there is no certainty that that’s the case – he just wants one tenant to deal with rather than 2)
Assuming you’re satisfied there is the intent to create legal relations between Miss B & Miss C (nothing to indicate that there isn’t), you could consider commerciality. Again, I can see nothing to indicate that it is not commercial, unless you know Miss B would not seek to revover rent arrears. Created to take advantage of HB scheme – again I do don’t think you can say this is the case.
I’m willing to be persuaded otherwise, but I think you should pay HB to Miss C.