They can take his blood because every state, including Montana has an implied consent law which means that a driver must test if the police feel it's necessary. It looks to me as if the police are screening for the presence of substances other than alcohol, which would be why they feel a blood test was necessary even though he passed the the breathalyzer.
There were DUI cases before the invention of the breathalyzer and a person can be charged with DUI on the basis of signs and symptoms that most people would be able to recognize as likely intoxication
. If the lab comes back negative for controlled substances, they prosecutor will not be able to sustain these charges, and the case against him will be released.
Meanwhile, if he was tased and does not believe it happened under circumstances that made such a police action necessary, he can lodge a complaint
with Internal Affairs and also with the Montana Attorney General's office. That will launch an investigation
into their use of excessive force , which would be helpful if he thinks that one day down the road, he might want to find a civil right's lawyer and sue them for brutality.
I have found nothing on the web to indicate that that the requirements of Montana's aggravated 2nd DUI law is unconstitutional.