You said this is "a good friend."
That her e-mail to you was about... your "eating issues", and then also, her friend told her about your "weight..."
Basically, THEY are critiquing not only your weight, but your eating and your appearance. And THEY have issues with it. And talked about it. Because then, your friend had to e-mail you about it.
But your friend, did not say... how she replied or retorted back, to her friends that made comments about you.
ie: if that were my friend, I KNOW, that my friend would have had my back. To that woman. They would have, displayed their friend/loyalty to me. And as such, my friends would have given that lady a hearty retort. Sort of like "well, that is her business, and its personal, you don't even know her or her life so you can't make a fair comment."
Since she is your "good friend" just tell her... it is none of their business, but also, I think your friend was just telling you what she heard.... since the others were talking about you. So she was "reporting" back to you, as a friend. But then, your friend, also... got on their bandwagon, and had to then get into it with them and then tell you.
So then, it feels like they were all gossiping about your personal life.
Which they were. ie: your weight and eating issues and appearance.
Icky, right? Awkward, for you.
At no time, does it appear... that your friend was backing you up. But it also seems, that your friend used this e-mail to tell you indirectly... about her or their "concern" for you... about your weight/appearance/eating issues. Maybe.... your friend didn't know how else to broach this subject with you... and thus used this situation as a spring-board, to then, tell you????
The point is: you don't know your "good friend's" intention.... unless you ask her directly and talk to her directly.
And since she is a good friend, I would think that you can be candid with her.
If not, then maybe she is not a good friend.