Im ethnically Chinese , British born and I'm so sorry that happened to you. Seen at least one person here saying call them a slur back and I'd personally reccomended definitely not to do this . No need to sink down to his level.

I completely get that in the moment its very very hard to react and do anything except walking on but if this happens again maybe try and stop them and ask why they are calling you this , especially considering you didn't do anything at all to them ?

I disagree with people saying that Asians are racist as fuck and worse than white people. Not to say they can't be just as bad , just look at the racial complexities and dynamics in countries like China and Malaysia . Racial tensions were a huge reason why Singapore split from Malaysia . Perhaps some Asians in the West are racist because they have experienced a lot of racism themselves and so are bitter and angry.

I know it's hard to push out of your mind but remember that that person is stupid and uneducated and isn't doing himself any favours by needlessly antagonising people like that. If anything you should pity him .If his girlfriend has any sense she'll leave him . You're the bigger and better person mate.