They're very protective of each other.
Who wasn't surprised when Sherlock's dad showed up? Naturally, a fiercely protective friend like Joan had to meet the man herself and size him up. She didn't mince words with Morland, telling him, "If you're going to be a part of Sherlock's life again, I want to make one thing clear. I'm not going to let you hurt him."
Watch Elementary's "Evidence of Things Not Seen" on CBS All Access.