My answer is (a). Am I correct?
a) Dawn received a call from a lawyer enquiring if their company was representing John and if they were, he requested her to send them a letter of representation
b) Dawn received a call from a lawyer enquiring if their company is representing John and if they are, he requested her to send them a letter of representation.
Both sentences are unnatural and too long. I would break "a" into two sentences.
Dawn received a call from a lawyer enquiring whether their company was representing John. If they were, the lawyer requested her to send them a letter of representation.
Who is the "they" in "their company"?
Dawn's company? If so, then it should be "her" company.
I can't tell if it's supposed to mean that the lawer who called (whose sex we don't know) is the "they" in "their company" but wouldn't a lawyer from that company know whether or not they were representing John?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.