The OP's friends won't even need a voucher -the National Trust of Malta lists all the places with which they have a reciprocal admittance agreement, and the Uk National Trust is one of them. - dinlarthelwa.org/about/reciprocal-membership/
If her card isn’t accepted here, it might be worth her while joining while she is here. It depends on how many NT properties you are going to visit. We went away with friends for four days, they joined NT at the first place we visited and by the end of the trip calculated that it had covered all the admission charges of the holiday.