Vets need to do a bit extra study when it comes to fish.. generally we call vets with aquatic degrees "wet vets".
A 1 gallon is suitable for a single betta, provided the tank is cleaned appropriately and heated.
70 is way too low - they can live in it, but in time it will have adverse affects on the fish such as lowered immune system, slow metabolism, etc.
Bamboo is fine partially submerged - you don't want the leaves in the water, but the stalk is fine.
Direct sunlight is fine too - as long as you don't do too much or are willing to clean the algae that will grow from it. Another concern is that it can cause fluctuations in the temp during summertime with the sun directly on the smaller amount of water, and in winter can cause the temp to drop being close to a cold window.
But if the tank is further from the window and sun comes through it at a distance, the direct sun is fine.
But I wouldn't blame the vet, they aren't always trained for fish - tend to have to call around and see if there is a wet vet practicing.. generally very hard to find.