Using European devices in America

Glad to help. Indeed, all Apple power bricks have been "universal" 94-240VAC 50-60Hz for a long time. The converter I was envisioning was to adapt the weird plugs used over there to good ole three-pronged NEMA used here.

The easy part is the mobiles. At the end of the day, all charge via USB. The question will be the plug at the end of the charger side, on the phone side (or the older iPads) all have Thunderbolt thus far while the charger side will be one of two: Type A or C.

So, your cables will fit his phone and your charging brick. His cables will fit his phone and maybe your charging brick: if older will be Type A, if newer will be the Type C. Use whichever fits. Which charger brick to use? The bigger available, your iPad bricks will happily work if the plugs fit with no harm.

For the MacBooks, there will be two possibilities: if the device is older (2017 or before), will come with a charger with integral cable ending in the MagSafe2 magnetic connector. Given they're both Airs, if it fits it works. Newer MacBooks now use USB-C and the charger has a separate cable with the same Type C connector (see above) at both ends. The charging brick to plug into has to have enough juice to charge the computer, the iPad bricks don't have enough.

If the BF wants to lug over his cables and bricks, he will need an adapter for American wall power sockets, usually available at Home Depot, etc. The airport Duty Free is the worst place to buy it, will be a legalized form of robbery.