(b) That's wrong.Some relationships are presumed to give the dominant person a general controlling influence over the other. These relationships include: parent and child; guardian and ward; doctor and patient; religious advisor and believer; solicitor and client; trustee and beneficiary; and most likely any relationship that depends on trust (a 'fiduciary relationship').
When such a relationship is proved to exist, there is a presumption that a transaction entered into on the advice of the dominant party is the result of undue influence. It is open to the dominant person to rebut this presumption. If they cannot do so, the transaction can be set aside as void, provided that there has been no unreasonable delay in seeking to have it set aside.
In the present case therefore, because A and B are solicitor and client, a presumption of undue influence exists. Unless A can rebut this presumption (by proving that B's decision to enter the contract was made freely, and not as a result of any such influence) B will be able to ask a court to set the contract aside as void. B will have to ask for this within a reasonable time.