The phrase "unexpected friend" is an idiomatic expression that refers to a person who comes forward as a supporter, helper or well-wisher at a time when such support was not anticipated. Full definition
A young woman named Alis takes up the quest to dethrone the king and avenge her brother's death, encountering unexpected friends and threats along the way. (psalgo.com)
So many unexpected friends and acquaintances are there. (housegoeshome.com)
So, my first real and extended experience with American evangelicalism came by way of this unexpected friend who shared the gospel with me. (christianitytoday.com)