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I don't think that counts as "stealing a gun"

Wouldn't it be "taking a war trophy"

Arrest vs detention can be tricky and my personal decision is that I will consider myself under arrest the moment any physical restraint is applied and act accordingly. I base my statement about handcuffs being "technically" an arrest on my understanding of the legal decisions.

This link provides a good summary and a good place to start a deeper research train.
http://www.legalupdateonline.com/4th/45

I am not a lawyer, police officer, judge or legislator and have been known to be wrong from time to time.

If the phrase "be arrested" was modified by any adverb indicating chance or probability like "may, can, are likely or even will", your instructor was informing you of what could happen. Using deadly force violates the primary statues against assault and homicide and you are subject to arrest and being charged. However the police may choose to not arrest and the DA may choose to not charge you. The police will almost certainly disarm you and technically, if they handcuff you, you are under arrest. What happens from there is based upon the evidence gathered by police including any statement you make and the judgement of the DA and/or the local Grand Jury.

It is good that your friend's preparation and actions allowed him to get through this violent attack unharmed physically and hopefully there are no lingering emotional injuries. This seems to be a clear cut and dried case of a valid use of deadly force in self defense as well as repelling a robbery so he should not have legal issues.