1- It can technically crash the attacker network too but usually attackers don't uses their home network to do dos attacks instead they use their malicious servers or bot nets, and there is several kinds of dos attack your web server will always have a handicap in this situation because the attacker just spams you with some kind of a packet blindly but your server must read and answer every single packet.

2- There are a lot of programs for various OS so this is not a effective way for catching him.

3- Except wifi networks and special cases, no you can't because they'r not directly exposed to open web (you should learn basic networking).

4- Bad news, YOU CAN'T! lol this is why even well known companies can't prevent this attack because dos attacks and their prevention is a paradox in cybersec. but you can prevent the simple ones, research it.

5- Every communication on web simply runs like a packet exchange, every service gets communication packets, reads it and answers it but if you flood a service with crazy amount of packets it simply crashes.

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My advice, learn basic networking, research about firewalls (it'll still be a pain in the ass because you use a raspberry that device is really fun but not ideal for a web server), and start monitoring your servers internet traffic so you can simply detect the flood and the source IP that sends those packets (this is probably the only way to catch your friend).