How can white people combat racism?

You did well to be diplomatic and leave. POSSIBLY, the guy was acquainted with them and was feigning participation with them, or even reasoned with them to not become worse. If you ever see the guy again you can probably gently enquire to determine if that was what it was. If really the guy was assisting the group of potato chip people. Or maybe they were hazing you in a way that to them wasnt considered very mean. If you determine that the guy was merely being disloyal to you, then consider that in future regarding the guy. Also, even if the guy was with them, maybe your Judo Spirit and kindness came back to you directly by them not getting worse, and maybe he told them to ease off on you because you were cool and decent. Or possibly the guy mentioned to them that they should learn breakfalling before having you demo osoto gari and o goshi and seionage and taiotoshi and harai goshi and morote seionage and ushiro goshi for them- free of charge, and in the Spirit of Judo, of course ;)