Taking that mob is unethical. You should be ashamed of yourself for even attempting to take it.
When the original gorup has zero chance of defeating they mob, no, they shouldnt be ashamed. that mob would just depop and anything it might drop would simply be gone.

However, if I were to be involved in something like this, I would consider inviting people from the group to lot on drops. Either way though, it is better that the mob is actually killed than no one kiling it, the mob depopping, and the pop truly going to waste.