TLQ and the suits in BP are wholly responsible for the AMW scandal.
AMW is responsible for the AMW scandal.
Alongside that, in a constitutional democracy responsibility is held primarily by those in government: in this case, cabinet ministers who appointed AMW to a government position and were responsible for his work in that position.
Secondarily, yes, his mother should have listened to advice. She was 75 when he was appointed to the trade envoy role, and it is documented by one of her close advisors that AMW could be very manipulative. Royal reporters have also documented that BP and York offices were very separate ('silos') so BP often didn't know what AMW was doing.
I've said above that I think she could have done things differently - including not putting him forward for the role. Once he was in post, the primary responsibility was with the government.