Back door route to top research schools?
I wouldn't call this a backdoor route.

It takes time to get quality publications and letters of recommendation yet undergrad goes by fast. Your friend also wasn't from a top university which makes these opportunities even harder to come upon.

There are a lot of details missing here (what were his letters of rec like, quality of publications, etc.) but it sounded like your friend did some good work and got accepted because of it. Whether you do this work during undergrad or after you graduate, doesn't necessarily matter.