On the flip side, I've been told by more than one peri and also read through my own research (years ago) that twins tend to develop faster (gaining weight faster, lungs mature quicker, etc.) than singletons. Not always, but sometimes. It's like their/our bodies know they may come earlier vs. a singleton, so the process is sped up a bit. Again, this isn't always true, but it can be for some apparently.
I agree that if she eats well (PROTEIN is so important, but just eating well in general) is vital in any pregnancy but especially a multiple gestation pg.
If I had not had other issues in my pg, my OB (who was pretty mainstream and all for induction and such, sigh..) told me he would have let me go to 41ish weeks before wanting to try induction as long as everything was going well. There's no reason twins (or beyond) would stop growing/maturing unless there was an issue (problem with the placentas or otherwise).