Engineering and the military
I agree with the previous comments that it will be very difficult to find an engineering discipline that cannot be used in war. I will make three further comments.
Regarding working for the military, or defense industry, countries have enemies and it is a duty of governments to protect their citizens. Working to maintain this obligation is an ethical occupation for citizens. This is not to say that all defense work is ethical and that governments should go to war without exhausting all other options.
The most promising fields of engineering that this student might investigate are biomedical or environmental. Both have the goal of improving conditions for people or the world, including restoration of damage caused by war.
Any technology can be used for good or ill; it depends on the mindset of the people developing it or using it. This student might find a career in developing peaceful solutions to conflict, or with an NGO such as Engineers Without Borders, Oxfam or UNICEF. I imagine that they all must employ some engineers in reviewing and assessing projects they undertake.