I think she will be completely supportive when you drop all that weight. Or, if she is also a large person, she is going to be very jealous! Either way, if she is a really good friend, like Missy said, be firm and let her know that although she may not agree, she could keep it to herself. You might let her know, that you have heard her side now, and she doesn't really need to continue to push the issue.
Something else i thought of, if she is also a larger person, she may already be jealous of your movements to make a better you, who knows. Best of luck in getting it all ironed out.