I interned thrice as phd student at FB. your friend isn't entirely wrong but also just doesn't have enough experience to judge. all enormous companies are like this. FB is far and away better than almost all such companies (probably only with the exception of Google/Netflix).
lol bruh read my response again - FB's and Google's and Amazon's tool are lightyears ahead of #ARBITRARY_F100_COMPANY. you haven't a clue what "bad" means if you've never worked in a place that has > 1000 engineers.
I'm not sure what your idea about PRs and features has to do with the above... he's not there to work on the internal infra framework. He's there for ML stuff. Unfortunately, the road to the later goes through the former, but he's not really a kind of programmer who'd deal with Facebook's infrastructure and plumbing.
The point is, it's inconvenient. Is it inconvenient because Facebook works on a five-year plan basis or whatever other reason they have for it doesn't really matter. It's just not good.
I also have no problems admitting that all big companies (two in total, one being Google) I worked for so far had bad internal tools. I don't imagine Facebook is anything special in this respect. I just don't feel like it's necessary to justify it in any way. It's just a fact of life: large companies have a tendency to produce bad internal tools (but small often have none whatsoever!) It's a water is wet kind of thing...
My idea is if he's not making the monorepo codebase changes himself, he's going to have to wait for an awfully long time for any non-trivial improvements he'd like because the responsible teams have different priorities sketched out for next calendar year. It's a function of organization size, unless you have the support of someone very high up on the org chart, ICs can't unilaterally adjust another teams priorities.
fork open source, then demand resources to maintian this monster.
easiest promotion + job security.
its even called "Platform Engineering" these days