Someone has said something offensive online.
You could remove them (or their post).
You could retort, try to offend them back, and get others to join in mocking them?
Or you could drop them a quiet line and explain “although you understand they didn’t mean to offend, some people are upset by what was posted because [reason]”
Which do you think helps the person the most?
This is less a question about the level of offense and more a question about the level of intent.
Did he intend to be offensive?
If the answer is yes, feel free to remove.
If the answer is no, have a quiet word and explain why it was offensive and what they can do to make amends.
Making people aware who unintentionally offend aware of their unintentional offense is a far better response than group bullying.