Emma wants to stop Harriet from marrying Robert Martin and she is using all the manipulative tactics to do that she can think of. What Emma is doing here is threatening Harriet with a more subtle version of the schoolyard "If you do this/don't do this I won't be your friend any more". Being somewhat older than six, of course, Emma dresses it up as "I couldn't be your friend anymore."

Look at the exchange shortly before the section you quote:
"Miss Woodhouse, as you will not give me your opinion, I must do as well as I can by myself; and I have now quite determined, and really almost made up my mind -- to refuse Mr. Martin. Do you think I am right?"

"Perfectly, perfectly right, my dearest Harriet; you are doing just what you ought. While you were at all in suspense I kept my feelings to myself, but now that you are so completely decided I have no hesitation in approving. (chpt 7)

Now Emma is smart enough to know that a girl who says "I have...really almost made up my mind" and "Do you think I am right" is not in fact "completely decided". In support of this, later on, we have the statement that:
... Emma believed if the young man had come in her way at that moment, he would have been accepted after all. (chpt 7). So Emma knows that Harriet is not settled, and that Emma could have swayed her to accept Robert Martin.

For this reason, and some others I won't discuss yet, I read this as Emma could be friends with the Martins, but at the moment it suits Emma's purposes to claim she couldn't.