So presumably the main part of the home is being heated, and the condensation is occurring in the bedroom when the warm air hits the unheated walls? And/or in corners?
The second can be solved by allowing air to circulate better behind furniture. As a house with radiators as opposed to gas heaters I am surprised there would be a lot of moisture in the air, unless she tends to hang very wet washing up all over the place (like a lady I knew).
She could have the bedroom walls overboarded to keep them from getting so cold, that would reduce the tendency for moisture to collect on them. It wouldn't be very expensive even if the insulating boards were used, and she might qualify for a grant?
As for the Legionnaire's - venting systems can nurture and spread this in a care home/hospital etc, but can't "cause" it in a one-person domestic setting!