Hi -

If sodium stearoyl lactyate is *sometimes* milk derived, but it is
found in a kosher meat or kosher pareve food, then that particular form
of it is NOT milk derived.  The laws of kashrut would prohibit it.  If
it is NOT in a kosher meat/pareve food, then people with low tolerance
for milk should certainly phone the manufacturer before trying it.

That's my take on the issue.
--Beth Kevles
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  http://web.mit.edu/kevles/www/nomilk.html -- a page for the milk-allergic