I would guess that a large deposit appeared in his bank account and that information was shared with the IRS.
Such sharing of information deemed relevant is now automatic.
As for who is applicable, the FATCA reporting start date applies to all accounts and investments in existence at 30/06/2014, and all of these relevant accounts are to be reported to HMRC annually by 31/5 following and exchanged with the IRS by 30/9 following. The first reporting calendar year is 2016, so the information will be received by the IRS for the first time in 2017.
The word ‘information’ could be considered a vague term, so to clarify, the information being reported on will include the following:
Date of birth & place of birth
Account balance or investment value
Sales proceeds of investments
Name of reporting entity
For those concerned about the use of their data, the Automatic Exchange of Information Programme includes terms that ensure the data is securely held and exchanged, is completely confidential and is only used to offset the problem of tax evasion.