I thought that someone can steal your coins only if they hack the wallet, have the private key, the wallet backup and the password.
That's correct.
I never thought that someone can just block coins in the blockchain or sniff a transaction somehow to end up with the coins you send.
That's not possible.
you're right, it's not -- so there must have been some additional component involved, I am thinking along the lines of Ransomware (those phone threats he did not shed any more light on, or similar...). Maybe someone got a backdoor into his desktop box (with his wallet on it) that way, that's how they actually got his private keys, too?
I would recommend you to stay away from such low value coins - especially master node nonsense.
The majority of them (probably 95%+) are a pure scam.
yes, stay away from low-value coins period. That's got nothing to do with masternode coins being "nonsense" though, or do you really believe you can discount Dash, PiVX and all the others just like this?! (Who are we today, Satoshi-on-steroids or somethin'...) Seriously.
Stay away from low value master node coin which could later scam investors have nothing to do with stay away from all master node project but due to the fact almost all master node coins go from grace to grass, the best way to advice a newbie is to stay away from master node project because newbies will not understand the features the look out for before choosing a master node base project.