A "trusted ally" refers to a person or group that is relied upon and considered reliable, dependable, and supportive. They are someone you can confide in, lean on, and count on for help and guidance. Full definition
The ideal company will both recognize that the author is in control of the book, and become your most trusted ally as you make decisions. (bookworks.com)
At most, we may gather the opinions of a few trusted allies on a given issue and then assume that everyone agrees with us. (ascd.org)
All those explanations are possibilities, but what is interesting here is that play - biting generally happens between trusted allies. (scientificamerican.com)