(In general, if it is in the core books (and if Wizards' GSL permits) it exists in Áereth ... after a fashion.
But first, a cautionary tale: Adrian, Chris and I were hanging out with Ed Greenwood at a habachi grill one night. I asked how he coordinates his personal Realms-lore with the dozens of sourcebooks.
This was his response: "I tell readers that everything I say is canon – up until something different appears in print."
So, taking my cue off of Ed "Elminster" Greenwood, here's what I'm working with, but understand that it isn't canon until it appears in print. )
Our warforged are distinct from the sort found in Eberron. I hope that's not what your friend is looking for.
Known Realms warforged hail from a specific time, and a specific war in the lost history of the Known Realms, when the sentient races were doing everything in their power to ward off a terrible foe. (Vague, I know, but you can find it in the Known Realms books.) One spellcaster, a master artificer, raised the creation of golems to dizzying heights, endowing his creations with sentience and --- some sages aver --- souls.
A total of 12 true warforged were created before the spellcaster was slain. (There are a number of theories explaining how and why he was murdered, but more on that later.) Of those 12, some have slipped in and out of history, appearing at key moments only to vanish again into the mists of time. And some have slept away the ages, only to awaken in recent years, answering some unheard call to battle.
Your friend's warforged (if he is still willing to play one) is one of those 12. Careful though. He has 11 siblings (some very sinister, weaving plots that span centuries) that have plans for him.
As always, grain of salt. Use what you like, discard the rest. We'll see if my warforged section makes it into the boxed set or not.