Dear Ross

Your friend might want to consider the design and fabrication of
metal-framed plexiglass storm windows which go onto a permanent metal
frame which is mounted on the interior of the existing glass windows and
utilize magnets for an air-proof seal.  In this manner, she could easily
remove and store the storm windows in the summer and the exterior
appearance of the house (fenestration) would remain unchanged.  I am
certain The National Trust For Historic Preservation could provide some
companies which do this.

Kind Regards

Roger Wulff
Museum Services International
[log in to unmask]