this is kind of the same boat that I was in personally...I mean I never stopped listening to (underground) hip-hop but it really tailed off for me around the same time as your friend when in the late 90s to like 2010 or 2015 it was just about the only music I listened to. I'm not sure what your friend was listening to before falling out of the loop, but I think for me the one album that really made it all click and got me back to listening to and exploring much more hip-hop outside of likes of RTJ, Bronson, Freddie Gibbs+Madlib, Danny Brown, etc... was Benny's Tana Talk 3. while I flirted with Griselda stuff before that a tiny bit that was the album that made it all click for me, so in my opinion I would definitely recommend that as a reentry point for your friend maybe above anything else. if he likes that hit him with Supreme Blientele, and WWCD, and some Roc Marciano, and Boldy James, etc...