Your brother needs to learn how to ledgetech. It's definitely one of the more difficult advanced techniques, and I still can't do it anywhere close to 100% of the time, but Falco's dair is relatively easy to tech. Just angle the up-B so you ride along the stage, hit R about a third to halfway through the firefox distance, and angle the control stick left and down. The autosmash dI is enough to tech the dair at lower damage but at higher damage you need to smash DI the dair, i.e., slam the stick toward the ledge during the stun frames of the dair. Also if you hold X/Y during the tech you will wall-jump which makes avoiding a second dair easier since you now have lots of options to get back on the stage. Your brother can also just try sweetspotting the stage but it's sometimes hard to do this.
What I do is the following:
1) up-B at an angle such that I ride up along the stage edge and sweetspot, if possible.
2) Prepare for the possibility I miss the sweetspot. This means I hit R a little after the firefox starts, hold C-stick down and left, and hold Y.
3) slam the control stick left and down when hit by the dair (or anything else for that matter).
By holding the C-stick toward the stage you will autosmash DI the dair, and by slamming the control stick you will smash DI. The C-stick is backup in case you miss the smash DI, so you will still get the autosmash DI which at some % is enough to save you.
