JUST A FEW POINTS
1. By itself, a cold water shower is invigorating in summer (high heat)
2. After qigong, it is best to carry on wth normal tasks to allow for more 'centralized circulation'
3. Just as the effects of running can have benefits beyond the initial session (24-36 hrs), qigong practice has a similar effect beyond the ending routine based on the type, duration and intensity of said routine.
4. Most people who do qigong tdoay may not be as physically conditioned due to modern societal goodies and degree of activity so frequent cold showers will weaken them.
5. TIbetans are a tough bunch! Compared to pre 1930's Tibet (let's say Kham vs Lhasa), some groups, based on environment have been known to be more resilient than others. Less people today may even want to go through Tummo training as part of their normal meditation retreat practice so the people teaching today may have less experience than those of the past, who actually had legions of students
my friend stated
I've got to go steal 6 yogas of naropa from my brother and read it - I've never gotten around to it.
Which version are you reading? About 20 or so years, I attended a course of Naro cho druk (Naro's 6 Yogas) with Glen Mullin, as part of an Introduction and he may still be doing this today. You might have to use your own google fu. A good idea is to get the book, read the background by Glem Mullin, which is more than just TIbetan Yoga. Different levels of introduction are part of it.