s.o. = someone

I have lived for at least a long while with the impression it is common abbreviation :D

It's a common abbreviation for significant other, i.e. one's husband/wife or partner, but never seen it abbreviate "someone" before.

"someone?"

this makes the most sense but i have never in my life seen someone abbreviate someone as SO, haha.

They're probably French, the French are always writing qqn and qqch for "quelqu'un" and "quelque chose".

A couple other small tells, although very fluent... "enterprise" instead of "business", etc.

Not exactly the same, but "sb." and "sth." are common abbreviations in dictionaries, e.g. "to meet sb." or "to pick sth. up". To those familiar with this convention, "s.o." can generally be inferred from context.