International reporting agreements outside EU

Not trying to evade tax, but curious if anyone is familiar with the practices around international reporting.

I have a colleague who had private (off-exchange) share trades in the US and Australia some years back. He waits that dividends had some withholding tax made, but not the proceeds of sales.

He has engaged a tax advisor to work out his retrospective tax liability but as he was paying tax in UK and Ireland at the time of these trades there seems to be some wiggle room depending on where he was deemed to be tax resident.

He wants to know how likely either the UK or Irish tax authorities will be made aware by the US or Australian authorities, or even if there is any mechanism to do so.

He’s trying to avoid the situation where he pays in Ireland and the UK come looking, etc.