'TO RUH' would appear to be 45 minutes then.
Online Check-in is available on bmi operated flights. This service becomes available 24 hours before the scheduled departure time and will remain open right up until 30 minutes before departure for UK domestic and Western European shorthaul flights, 45 minutes prior to departure for all other routes.
If the OP turned up 42 minutes prior to departure that would make them too late and the airline is correct.