I didn't know the programme, but I've found the bit I assume you're talking about on YouTube. It's part of episode 1 and stretches from about 2:06 to 3:45. The video is low-quality and needs playing at aspect ratio 16:9 to unsquash it (couldn't find a better one), but here's what I could make out.

Right at the start, I think he protests "I had a forcefield!", and there's the sound of his shield recharging. He seems to be on level 2. That's most clearly seen from around 2:52 where you can see that he's in the lifeboat area at the start. But Marines are heard, which is wrong for that place of course. Cries of "My fault", "Watch your fire!" and "I had a shot!" suggest a pretty congested situation, and there are sounds of Elites and Grunts too. Evidently this is some separate audio added later.

In some of the footage he appears to be firing an AR at nothing - quite a lot. At one point he's firing an AR directly up at a dropship (which is fine, albeit useless). At that time you can simultaneously see him doing stuff with the controller. I doubt that he's actually controlling MC there, but the footage is only brief and a bit fuzzy, so I didn't bother to look closer.

Two types of Halo music feature, both inappropriate. One bit of that (starting about 3:17) is the music you trigger with Marines in the spiral path area of AOTCR. A Marine is heard to say "Nobody on this side".

One good thing; he does have a believable absorbed gamer's stare.

Note: A snippet of programme footage at about 2:11 (showing a wide view of the room, featuring the player and the TV) is actually repeated around 2:59. A few needles go across the player's view.

: No after-editing, just footage of the game and audio as presented by
: the TV it is being played on (unless my memory is FALSE!).

Ha ha - memory a little bit "FALSE!" then, I'd say. As mentioned above, the audio seems bogus. Maybe some of the correct audio is there too, but I'm not sure.