Regs were finalized in July of 2018 (I think that's when it was) allowing for that change. The original position always seemed strange to me, the change was a welcome one.
Opening balance: $A
Deferrals: $B
Match: $C
Distribs: $D
Total plan earnings: $X
Earnings basis: PARTICIPANT [BOY + .5(contribs) + adjs - distribs] / Plan [BOY + .5(contribs) + adjs - distribs]
Total participant earnings: Basis * Total Plan Earnings
That is all the attorney needs.
Update on the facts:
The distribution was for $220,000.
There was no money source (yet) as the money is in a brokerage account for the participant. The brokerage does not separately record keep the sources; we do that annually. She had enough funds in the PS account at BOY to support the distribution.
I believe the money is still in the IRA, so re-payment may still be possible.