6 weeks is too young for a kitten to leave its mother, no matter if its been
eating on its own.
It is customary for non-pedigree kittens to be moved to their new homes at
about 8 weeks of age, while pedigree kittens are often not separated from
their litters for re-homing until 4 or 5 weeks later.

If you take a kitten away from it mum and litter mates you are stopping it
from learning to be a cat an socialising, a lot of kittens are still feeding
from the mum but taking solids too.

I would suggest she keeps them till they are 8 weeks old

Yes, without a doubt. Kittens should not leave their mother until *at least*
8 to 10 weeks, 12 would be even better.

Ivor

Yes, he's too young. Wait until he's at least 8 weeks, 12 weeks is best.
There's more to weaning than eating solid food. Kittens need to learn
proper socialization from their moms.

I HIGHLY recommend keeping the cat with its mother for a few more weeks.
Granted young kittens are absolutely adorable, but if you have no experience
with them, they are a total pain in the tushie to train, to socialize, and
to clean up after.  Yes.. they know how to use the box, and in general will
learn to groom themselves in time, but at 6 weeks they don't.  and they tend
to walk in their food, and through their feces, and then walk around the
house like that.  They also have no idea that biting and scratching is
unacceptable.. so they tend to do a lot of it, especially since they are
teething... which they all do.  along with a number of other issues.

Having fostered kittens for two years now, Ive had kittens that had mothers,
and ones that didn't.  It is feasible to take home a kitten that young, but
your not doing it or yourself any favors - unless you seriously know what
your doing.

I adopted a 4-5 week kitten earlier this year and I feel that it was
way too young.  Sadly, it died a week later of a probable brain
hemorrhage.  A month or two later I went back to the animal shelter
and selected a 10 week old kitten and it has worked out wonderfully.
Kittens are theoretically weaned at 4 weeks, but in reality should
continue to nurse for a while after.  They are not psychologically
ready to be removed from their mother yet.  The first kitten was
literally a helpless baby, she couldn't bear to be left alone for a
microsecond.  I'd ask the coworker to hold on to it for another 2
weeks.