Tom,
You probably remember the Finnish study showing that a short 9 week Herceptin treatment had lasting effect. So a relatively brief interruption may not have serious consequences. The other case to recall is that of Gina with her repeated ON/OFF treatment cycles.
When your mother resumes Herceptin or goes eventually to Tykerb, it may still be usefull to go on with Herceptin even if resistance sets in (hopefully not so). As you may recall several oncs have commented that Herceptin is still beneficial after progression at least in some instances (Ref.2). I do not know not for sure why this is so but one of the reasons may be its multiple antiangiogenesis modes of action that have been discovered in the past:
Ref. 1
http://www.oncolink.com/resources/article.cfm?c=3&s=8&ss=23&id=8198&month=03&year=20 02
Herceptin reduces the expression of at least three angiogenesis-related genes and upregulates an anti-angiogenic factor
Ref.2
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=128204 39&dopt=Abstract
The effect of trastuzumab/docatexel combination on breast cancer angiogenesis: dichotomus effect predictable by the HIFI alpha/VEGF pre-treatment status?
This, on the surface, but not in fact, looks analogous to the remaining antiangiogenesis mode of action of low dose chemo after cytotoxic high doses have become useless.
However one of the uncertainties of both Herceptin or Tykerb prolonged treatments is the possible shift, in some cases, from HER+ to ER+