It's one possibility, and probably the most likely if it was once fired brass.
However, in the mid 1980s I bought a large quantity of some new TW73 head stamped brass at a local gun shop. I wasn't smart enough then to ask why the military had surplussed virgin brass in 1973.
On a prairie dog hunt a bit later, I was shooting 55 gr varmint loads in that new brass in my Varmint AR (back in the day we milled the carry handle off of an A1, then D&Td it for a weaver rail, just in case you wondered where the Picatinny rail idea came from). A couple hundred rounds into the day one of the shots sounded funny and I noticed the contents of the magazine were rolling around in the dirt under me (bipod prone position) along with the follower and spring. I noted that the portion of the 20 round magazine that was below the magazine well was blown out to the diameter and shape of a beer can.
The bolt would not retract, and on further inspection I noted the bottom of the bolt carrier had cracked at the front with the piece bent 90 degrees down into the magazine well. Which is a pretty effective way to lock up the carrier on an AR.
It took a hammer and punch to drive the bottom of the carrier back into position so that I could retract the bolt. At that point I could also see that 3 of the bolt lugs were basically melted off and 2 of the remaining lugs were cracked. The extractor was gone.
The head of the cartridge case had completely failed and was also missing over about 60% of it's diameter.
The damage however was minimal as I got a new bolt carrier and bolt assembly for around $100, checked the head space and put it back to work, accurate and reliable as ever.
I was impressed with the AR-15's ability to handle a massive case head failure.
I didn't note any gas or pieces of metal coming back through the charging handle, but then I was wearing shooting glasses - even in the mid 1980s that was a more or less expected safety measure.
In your friend's case, I doubt the piece of metal came that way, but rather was probably blown that direction by the escaping gas outside the ejection port, or was deflected off the receiver or port cover.
Eventually, after some extensive research I found that the US military had surplussed a number of lots of virgin TW73 brass for scrap due to concerns that some of it may not have been properly annealed, with the result that the case heads were soft.
Some of this new brass apparently found it's way into the market as reloading components and I got one of those soft headed cases and it was in no way strong enough to contain 50,000 PSI.
