I'm assuming they're using the 18-55 kit lens, since that's what comes with the camera in the box, and if they didn't have that, then they wouldn't be shooting with anything, because that assumes they only bought the camera body.
A 50mm 1.8 prime lens is a great investment. I bought the same one. And for that T3i, you can't go wrong with having it.
Although, if you definitely want to start shooting something with sound with regards to that camera, then you probably should go for the Rode mic, no matter what model it is. Anything is better than the internal mic on the T3i. It is total garbage.
I moved up to the Panasonic GH4 last year, and the internal mic on that camera is far superior and pics up sound far far better. It isn't sound you'd want to officially use, as it's still very crushed and it peaks out easily, but at least it isn't as tinny or as soft as the T3i is. You can also hear stuff almost in 360 degrees with the built in mic on the GH4. So for run and gun purposes, you probably don't "need" to get a mic right away with it.
But for a T3i. Yes, absolutely, I would go with the Rode mic as your next purchase. The 50mm can wait for a bit longer.