What you know is related by your colleague to you, you didn't
hear the boss scolding her first hand?
The tone of his voice matters. Also there is a chance for words to
be twisted out of context.
Is there someone in the hierarchy between you and your GM? If there
is, it is more his place to speak to the GM.
If there isn't, and you report straight to your GM, you can still
do something.
Find a time to talk to him in private, like in a "by-the-way, just
to keep you updated" manner, tell him that your colleague has been
feeling very down , depressed, lost appetite, etc. ever since he
scolded her using such harsh words. And that what he did hurt her
(and everyone who had been scolded before) feelings. And that you
are concerned for her well-being and health.
Don't expect an apology, don't push for any admission from him, but
hopefully he will tone down in future. Do not be
confrontational.
If I were insulted, I'd immediately say, "I may have screwed up and
I apologize for that, but there is no need to call me names and get
personal".
