My friend is going to try coming to my corps with me. But one of his questions was, what makes the Salvation Army different from other Christian churches. Do you have any suggestions on how I could answer that question???

Great Q! Your answer can vary depending on what community/province you live in, what your local expression is like when it gathers, what your friend means by the question (how is the service diff't OR how is the body diff't), what 'other churches' they are referring to (Catholic, Baptist) and what their faith background or understanding is.
Assuming they're asking about the 'Sunday gathering' then you might talk about having a brass band (depending on your community) and members wearing uniforms.
Assuming they're talking about the body, one thing that is pretty different from many others is that we have a particular purpose by God for the marginalized of society (heart to God, hand to man; soup, soap, salvation) and we don't practice communion and baptism as part of our gatherings.
There's much more that could be said but again, it depends on your context. Hope this helps?! If not, post another question being more specific or fire us a private message!