We don't yet know how he made that much money. If he coerced or tricked women into prostitution, then he is scum. But he might not have done that.
According to the UK's mingbogglingly stupid trafficking laws, consent is not a factor IIRC: arranging the transportation of willing WG's is still a crime. And as I've pointed out elsewhere, taking 50% of her earnings is not unreasonable if Sergei is paying for all expenses out of that, including hotel bookings: many British independent WG's would probably be happy to make a guaranteed profit of 50% of all takings after expenses. On a good day, she'd make a few hundred: on a bad day, Sergei is the one who loses out.