Loose coupling, accessing class properties
As DataRow does not implement IDataRecord, you would have to pass an adapter that wraps the row and implements the interface. Also, passing an IListSource would only help if it always returned a ITypedList (which is not guaranteed) which you could then query for available properties by name. But what would be the advantage of all that?

In my opinion, the parameter types are alright. We are still talking about a private helper method, not one that needs to be accessible using all kinds of interfaces. There are, however, some things that I might change:



Either: Rename the method to represent that it actually does not care about the legal first name. You could currently rename it to ContainsLegalLastName without any change in behaviour.
Or: Remove the legalFirstNameColumn and get that value from somewhere else (some configuration, const, …).

Unless you decide to remove the column name parameter and get the column name from an instance variable, make the method static. It currently does not use any state at all.