Not sure why there needs to be a 'standard' as such. Any well designed UI will give the user options regarding how denominations are displayed.
A comma is a traditional thou separator, both in maths and finance, for numbers > 100
I (for one) would not want numbers < 0 displayed with any separator.
The issue is that comma conflicts with decimal separator in many countries (especially in Europe, but many others).
Apostrophe works better: 0.03'445'674 BTC
B,BBB.ss_sss_sss
Examples: