Joining a Small local domain to a larger domain

We have a new site that we’re going to support. They have a server and local domain, and while we can see and access the server (dameware, Remote desktop) I can’t create a trust relationship so that I can bring them into our domain.

How do I adjust their DNS settings so we can set up the trusts? I don’t want to break anything on their end, but I need to move forward on this.