I'm not sure about the wheels, but the total load being driven, so car being loaded and trailer being loaded can't exceed 3500kg for your wife to drive with the trailer on her current license.
In all honesty a family car, plus the loaded trailer isn't going to exceed 3500kg, but for all I know you might be driving a L200 or something!
She'll be driving a 2011 vauxhall Zafira which has a Gross weight of 2090 and 2175kg depending on engine , not sure of the weight of a fully loaded trailer is