The entire argument is bogus.
The beliefs of a pacifist are valid - for them. The beliefs of a non-pacifist that the life/well-being of even a pacifist are worth defending from the threat of death or serious bodily injury are valid - for the person so beieving.
The conflict between the beliefs of the pacifist and an intervenor (go look it up) only gets to be sticky when it is the government intervening. But there are caveats even to that. For example, the government cannot force someone who has expressly identified themself as a Seventh Day Adventist or Christian Scientist to receive a blood transfusion when that person has objected on religious grounds. Even if failing to give te blood transfusion is 100% guaranteed to result in the death of the Seventh Day Adventist ot Christian Scientist. But unless that Seventh Day Adventist/Christian Scientist carries some recognizable indication that government agents (a police officer, for example) can recognize/understand even at a distance that government intervenor cannot be held liable for violating the First Amendment rights of said Seventh Day Adventist/Chriatian Scientist when they shoot/kill someone whose actions/behavior are a recognizable threat of death or serious bodily injury.
Cutting through the bureaucratic-ese - at the bottom line your personal beliefs can not and should not have absolute control over the exercise of my beliefs. What should be the controlling factor is my respect for your beliefs. That's where things get sticky - do I live for the rest of your life with you being displeased with me because I failed to honor those beliefs you have taken pains to inform me of, or do I live with some notion of guilt due to being responsible to some degree for your death?
Now we get to the first level of legalities - does the law "allow/permit" me to intervene even if I am fully aware of your personal beliefs? Generally speaking the answer will be "Yes".
The second level of legalities runs generally along this line - have I committed a tort you could seek compensation for if I intervened when I knew of your personal beliefs? Beng unable to find any case law for guidance I can only guess. My guess is "No" based on the concept that you cannot force me to adhere to your beliefs. (And besides, if you were a true pacifist you could not hold the notion that you could force your beliefs on another.)
Now I can get to the OP's question: " How would [I] feel about defending a 3rd party, a friend, if you knew that they would not use lethal force to defend themselves, and would not want you to?"
Frienship carries with it some level of respect for the beliefs (no matter how irrational) of the friend. Yet it does not, IMHO, impose an absolute duty to adhere to that belief system. Addressing the possible irrationality of my friend's beliefs - how far out do they extend beyond themself? How much does adherence to those beliefs, and forcing me to adhere to those beliefs, impact on society? As a quick example - suppose my pacifist friend has a minor child. Will their death impose a financial burden on society to be involved in raising that child to adulthood? Should society be required, as but one example, to pay Social Security survivor benefits (which would not be paid if the parent were not dead)? What emotional impact will the parent's death have on the development of that minor child? Will it become a criminal for lack of parental guidance, or because of the bitterness expressed by the surviving parent at being forced to raise the child alone? Will placement in foster care create a delinquient/behavior problem that grows up to be a used car salesman or - worse - a telemarketer? Or will the death of the parent be the motivating force behind the surviving child discovering the cure for the common cold and an end to embarassing dandruff?
Then again, let us suppose the pacifist parent were, in addition, psychologically "messed up" (a precise clinical term) such that had they lived their child would become another Barak Obama?
We could play this game until one of us (it would be you, because I can keep this up all day) ran screaming from the room and still never get to a decision that would serve as an answer to fit all occassions and situations.
I like to think that I know myself well enough to come up with an answer that will work for me. I'm not going to share it with anybody because it ought to be meaningless to anybody except myself.