Gness;
Q: “How many people have to be offended before it becomes a racist 'act'?”
A: One.
Recommendation 12 (of 70) of the report into the death of Stephen Lawrence:
DEFINITION OF RACIST INCIDENT
12. That the definition should be: "A racist incident is any incident which is perceived to be racist by the victim or any other person".
That recommendation was accepted and has been implemented. So, if the lady who took umbrage on the tube believed she witnessed a racist incident, it was a racist incident.
From;
https://www.theanswerbank.co.uk/News/Question1630026-2.html