On 120k you can get your own apartment in NYC outside of Manhattan and the expensive parts of Brooklyn easily. Plus that is for a new grad / jr engineer. They’ll be making north of $200k in 5 years most likely.
Assuming you pay $200/month in healthcare, ~$1800 a month to max out your 401k, your take home pay is about $5,000 after taxes. In Manhattan and the expensive parts of Brooklyn, your own place is realistically a minimum of $2,500, possibly as much as $3,000. Outside of those places, still $2,000.
Source: I literally just went through this process, as a new grad, making slightly above $120k about 3 months ago.
Plus with junior devs the salary increase is pretty steep in the first few years.
I think the argument “he has to live in a share flat otherwise would need to compromise on maxing out with 401k” isn’t as convincing an argument as the OP thinks.
What about the cheaper parts of Brooklyn, Queens, the Bronx or Staten Island...? Not even to mention Jersey City.