Problems with the CARES harness for a kid that age:
1. If you need a car seat on the other end, and don't have one, you have to check the car seat. I have seen how gate checked seats are treated, 2/3 of the gate checked seats I check afterwards have visible damage, and I've had gate checked items (strollers) lost. So it's not something I am comfortable with with my own kids or advise people to do. If they have an alternate plan, though, this problem isn't a problem. So this one doesn't apply to them.
2. Not all kids fit the lap belt at that age/size, it depends on build and belt, and the CARES doesn't tighten it at all. When my kids were 3.5/37 lbs., 5.5/45 lbs. and 7.5/45 lbs., my younger two fit the lap belt fine (it sat low and tightened enough) because they are built stocky, while my oldest, who weighed the same as my second, did NOT fit the belt because she is built long and lanky. We had not brought a seat along and there was nothing we could do. You could pull the belt 3-4 inches off her lap when it was fully tightened, and she was very nervous like that, and so was I, and there was nothing we could do about it. She slid around during normal ascent and descent, I shudder to think what could have happened if we'd had any turbulence or sudden stops.

And there's no crotch strap so they can slip right under. If kids are borderline on height for it (my 3 year old was under 40 inches but had a tall torso and was riiiight at the limit) it could pull the belt UP off the hips if you are not careful, too.
3. Kids can unbuckle. Not a problem for all kids, but my particular 3 year old, though generally well-behaved, got upset several times (being overtired and cranky) and unbuckled on our flight out using it.
None of these problems are insurmountable. Since they have a safe seat on the other end, as long as he's not built long and skinny like my eldest he'll probably fit, and she'll just need to be vigilant that he doesn't unbuckle and have distractions ready if he gets cranky.
It DID help with sleep support for my 3 year old once we finally convinced her to take a nap.