Putthat, I'd second telling your friend to book it anyway. They didn't even query dd at 18mo and I know my friend squished her 23mo into one .
TBH, they're just a handy thing to have during the flight as a general dumping place or somewhere to sit lo while she plays. Having a wriggly toddler permanently on your knee for 24hrs is not fun!
On the longhaul flights they are put up as soon as the seatbelt light is turned off after take off and stay up until you start the descent into your destination. The only time we weren't allowed to use it was during turbulence when they had to sit on our laps, strapped in. It's a bit of a PITA if they're asleep at that point but the cabin crew weren't that strict on it.
I can put a pic of ds2 in his on my profile if your friend wants to see it!