Definitely not "girly conniving", you certainly are "just a bloke". How about "emotional support and feminine solidarity" although, come to think of it, "solidarity" in the concrete sense of the term does not quite go with slimness!! Any brighter ideas out there?

a secret (understanding) between women

Is the phrase being used to describe the actual ad (i.e. she winks smugly & conspiratorially at her equally slim friend) ? or is it feedback about how 'emotionally engaging' this conspiratorial approach
is ?

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Note added at 1 hr (2008-02-01 14:18:37 GMT)
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i.e. exploiting the notion of 'female togetherness'

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Note added at 1 hr (2008-02-01 14:36:12 GMT)
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therefore I'd go for ' (the ad uses an emotionally engaging) conspiratorial slant between the women'. Unless she looks at the screen and gives US a wink and there is no rapport shown with anyone in the ad itself.

That is a quite widely-used term for it.

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Note added at 1 hr (2008-02-01 14:36:20 GMT)
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Or "gurl talk". (I put this note in earlier, but it disappeared).

emotional support and understanding from other women.

We also keep a mental card in the head which can get marked for years, by misrepresentations of us........ ;-)

Have a look at Weightwatchers or something - I'm sure they'll have all the answers you need

French term (edited): via complicité féminine

I realize that it is probably just me, but I'm a women and my gender is female. Many species have males and females, but only a human being can be a man or a woman.

or is that going too far? Complicity is defined in Chambers as the act of being or an accomplice and/or complexity, so this may work.

Here's a radical idea. Why not drop the whole "emotional support" thing and go for the "complicité" factor:
"A whisper from your girlfriend: the secret to staying thin!"

This addresses the ultimate customer and makes the whole thing sound slightly secretive and mysterious without being stilted.

If this translation is too free for you, maybe you could use the word "network" to convey that enigmatic (and, for us blokes, ever-so-daunting!) feminine "complicité":
The girls' network: emotional support from a slim friend

Remember: Thin girls/women don't let their friends eat fattening food... ;-)

But what do I know? I'm just a man...

I'm not kidding.

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Note added at 6 hrs (2008-02-01 19:05:53 GMT)
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Well, just Google the exact phrase is the link (in reference to Almodovar's "Volver") doesn't work.

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Note added at 9 hrs (2008-02-01 21:51:23 GMT)
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The primary meaning of 'complicity' in *French* as well as in English is intentional participation in a wrongful act. The other French meaning is "entente profonde, spontanée et souvent inexprimée (entre personnes)".

This expression and meaning has already been adopted in English in lit-crit and art-crit circles (cf. review of Volver linked above). No doubt part of its allure is the connotation of secrecy and masculine exclusion. Maybe your job as translator is to just to advance its cultural penetration by choosing the obvious translation.

OTOH, if you think readers may understand what it means in personal ads, you may want to avoid it.<g>

Tend tyo agree with Paul Cohen's line of thought.

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Note added at 9 hrs (2008-02-01 22:00:11 GMT)
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Or how about "Between Us Girls"