If the LHR-MAN sector was booked into (Domestic) Premium Economy fare then she should have had access to the Business lounge. If it was just booked as Economy then she wouldn't.
http://www.flybmi.com/bmi/en-gb/plan...&route=LHR-MAN
premium economy and business passengers can use bmi business lounges and some third party lounges (please refer to airport information to check where available).
I'm a bit surprised that if booking a C award with a BMI domestic economy only cabin involved, they don't book that sector as premium economy.