What should a fasting person do if someone starts a fight/quarrel with him?
And the servants of the Most Merciful are those who walk upon the earth easily, and when the ignorant address them [harshly], they say [words of] peace (25:63)
If someone verbally abuses a person who is fasting, then he should not respond by swearing back. Instead, he should say: I am fasting.
Akhlaq e Hasanah (good akhlaq) urges us to respond evil with goodness, instead of responding with evil.
Things to do:
If someone quarrels with a person who is fasting, then it is better to have patience and to not fight back. One should respond by saying: I am fasting.
Prophet of Allahﷺ has forbidden taking revenge while fasting.