I have seen some other people mention Airscreen for Android although I've not tried it myself I think you can download it as an app, the free version has various limitations but would probably give you the chance to try before you buy it.
If you ended up having to buy a box and don't want to spend Apple TV money, I've just bought a Sunvell T95Q which has
AirPin (Pro) installed on it ... not sure if that's available as an App or not.
I've only used it portrait for photos so far but if you had any interest can test it this evening and will try with videos and sound.
Actually I just read your post again, why is you Mac better than your Android TV????
I bought the T95Q mainly as my TV was so incredibly slow / clunky for catch up, this has made it so much quicker and easier using a mini keyboard and you can download them straight to the box if you suffer buffering with your connection.
I used to get loads of buffering which I thought was my internet connection but since I've bought this have realised it was the old Android that was causing it not the connection.
He needs to use a VPN so he can stream outside of the UK.
His smart TV may not be an Android TV. If it is he may be able to install an Android VPN app and use it without the MAC. If not he needs to use another device.
A Chromecast should work fine. https://www.lifewire.com/how-to-chromecast-from-mac-4173974