"Whatever's going"= you don't really care as you agree with whatever your friends are doing

In British English, "whatever's going" is a valid usage to say that you will take whatever you are given - i.e. whatever is on offer. "What would you like for dinner?" "Whatever's going!"

The short answer to this is...well, you don't say it. I'm sure in some context it may make sense, but only as part of a conversation where this would be some abbreviated response. Going, if used in any sense other than as a future form or literally 'moving from one place to another', will most likely be in the form of a 'phrasal verb'. That is a simple (normally monosyllabic Germanic verb like go, do, make, put, set etc) together with a preposition or adverb. The meaning then becomes unpredictable from it's original one. So, you can certainly say "whatever's going on" meaning 'happening', or "whatever's going around", meaning 'something (usually an illness) being passed from one to another', or "whatever's going down", meaning something big and important is occurring right now. But, without that second part to the verb 'go', it does not make complete sense in this phrasing.