I would still like to know the source of this statement, but it does not appear to be the Bodhisattva-bhūmi. According to an electronic search of the digital version, the word pañcaśīla does not even occur in this text. So it seems that the author of the Bodhisattva-bhūmi did not make any statement about the pañcaśīla, let alone a categorical statement that a devout Buddhist may deviate from these precepts.
The digital version of the Bodhisattva-bhūmi
(Dutt edition) is available at the Digital Sanskrit Buddhist Canon
website (http://www.dsbcproject.org/), and also at the GRETIL website
(http://gretil.sub.uni-goettingen.de/). Page image scans of the two
printed editions, those by Unrai Wogihara (1930-1936) and by Nalinaksha
Dutt (1966), were posted by me on the web
(http://prajnaquest.fr/blog/sanskrit-texts-3/sanskrit-buddhist-texts/).
Only a few months ago the first complete English translation of the Bodhisattva-bhūmi was published: The Bodhisattva Path to Unsurpassed Enlightenment, translated by Artemus B. Engle (Boulder: Snow Lion, 2016).