The walls were not insulated and would sweat in the heat, making moisture in the walls (perfect for termites). 

Which means an upgrade to windows and insulation is a must to keep your tenants happy without them paying 500+ a month to heat their rental in the winter.

We insulated the sill and headers, then wrapped the pipe and there have been no further problems.Is it a difference in building techniques (insulation, location of plumbing runs, etc) which contributes to the seemingly high incident of frozen pipes?

@Matthew Paul Or address the ventilation and insulation problems in the attic and there will be no ice dam.Every winter I'm amazed by the number of frozen pipes and sewer lines in places like Md, Tn, NC, Virginia or even Pa.  

I like putting in new windows, insulation, landscaping, ceiling fans etc.  

@Irma Hermida Medical students, for sure something like well insulated dorms - you know how pressured they are on their studies, perhaps a little business arcade/business centers.

She thinks the insulation is saturated with mouse pee...

Before:Later:After:This house is extremely well insulated with 2'' of exterior foam board under the siding and an air sealed and insulated attic.

She does need new A/C unit, insulation and possibly new windows and some structural work, plus cosmetic repairs. 

Fix soft spots in floors (3/4" osb)  Windows - caulk and plexie glass, Roof - rubber, if exterior is trashed new siding on ferring strips, and white paint interior, carpet (no pad just glue).Make sure 3 things work perfectly: electrical (may need to update), plumbing (pex lines with heat tape wrapped in pipe insulation, and furnace (don't slack on this).These tenants want to move and thins to work.