If the bf can understand what the gal is going through, i think
he will empathise with her. Validate her feelings and help her
through this situation and not to get angry with why she allow the
other man to do this... like u said its a not so normal situation,
she need to think for the best solution and she don't have time for
that back then.
Since it has already happened, so its time to think to prevent it
from happening. Like what others have said, use body language...
else to protect herself, she will need to make it more obvious and
with all other associates around, he should back off.
In order to gain something, one needs to give up some other
things.
