Does anybody else find this strange? There's this person whose name you don't even know, but somehow you know who his old friends are? This is not a situation I'm familiar with.
You left out the adverbial phrase. The whole sentence is
> When he reached out to my company six months later to apply for a job, I learned his real name and used it to track down an old friend of his to ask for help — but the friend told me he was afraid to intervene because he didn’t want to become a target himself.
When the stalker applied for a job, additional details may have become available to the OP, potentially including personal references (i.e. "old friend".)
The sentence is a bit ambiguous but that's what seems to make the most sense to me.