Liking the post that mocks your friend is still taking a side.

A tap feels like nothing because it is anonymous in your own head and visible to everybody else. It adds a number to something aimed at somebody you know, and that number is the whole mechanism by which the thing spreads further. Your friend will see the list. Nobody reads a like as neutral, least of all the person being mocked.

Scroll past rather than tapping, which costs you nothing at all. If you already did it, take it off and say something to your friend directly. And if you think the joke was fair, say that to them rather than to an audience.