I have a very negative opinion on Javier. IMO he has contributed little to nothing to Khabib's career and is essentially there for show/ because Khabib wants to be "loyal". I think Khabib's success is 50% his father(genes, nutrition, early life, making him do sports, wrestling, advice, gameplanning etc.) 50% dedication. Most of the AKA success was from big guys like Cain, DC etc. training there not from Mendez. That's why Rockhold didn't really care for staying there. He wanted real coaching input/analysis.
From watching all the Khabib vlogs he seems to barely ever train Khabib or give him any insight/advice. Khabib barely even uses his striking anyways. I believe he's just sticking around because he wants to get a large payout from his % cut of the fight.
Imo Javier also doesn't like Khabib that much other than him being champ, they don't get along that well due to language barrier and Khabib always talks russian with the rest of his camp which Javier doesn't understand so he's just sitting there like a mute. Doesn't give much advice in the cage either. Whenever I watch them interact Mendez seems like he's annoyed. Which makes sense since Khabib just talks russian 90% of the time unless DC and co are there.
Imo the relationship is strained.
Also I don't believe I've ever seen Mendez take any credit whatsoever for coaching Khabib b/c he knows he has little to do with his success.