Question Author
Thanks everyone. A few points:
First of all, sorry I mistyped her age - it's 80 not 90, although that isn't too improtant.
Second, I tried to ring Sky for her but without a password they wouldn't allow a conversation.
Third, Mrs A has already written to Sky explaining it all but they wrote back saying that as the account wasn't in her name they needed the mysterious Mr B, and only Mr B, to contact them as the account holder.
We could try CAB so thanks for that suggestion, or maybe an MP.
One approach may just be to bin/return to sender all future correspondence. Fortunately the account doesn't seem to be in arrears at the moment, so maybe we should just wait and see if the problem goes away. We know that if the account gets in arrears it could become a problem for Mrs A if bailiffs come round but we could cross that bridge when we come to it.
Thanks for your help.
Any further thoughts would be welcome