That's simply my suggestion because I've never actually come across an English equivalent. I've had a look on Google and no English proverbs came leaping out - not with the devil and a woman as their protagonists, anyway.

However, I found this variant (which is given a Polish attribution):

'When the devil is at his wits' end, he sends a woman'
(http://tinyurl.com/2j694a )

And this one (which is Ukrainian):

'If the devil is powerless, send him a woman'
( http://tinyurl.com/3xuo4l )

Here's one from Germany:

'Where the devil can't go himself, he sends an old woman'
( http://tinyurl.com/3bsqpl )

I also had a look on several English proverb sites and there was nothing similar - all of which leads me to suspect that there probably isn't an exact English equivalent.

Personally, I would use the variant with 'fears to tread' and this brings it closer to the English proverb 'Fools rush in where angels fear to tread', which gives it a kind of linguistic echo of veracity. However, it depends, of course, on the nature of your text.