Yes, a hard fork is different than a 51% attack. A 51% attack would use the current rules of Bitcoin Core (including the 21mil cap) to gain the majority of hash rate. This would allow them to validate fraudulent blocks to the chain, but under the current rules so other nodes (not part of the attack) would validate them. They could only steel or double spend coins in recent transactions as they include them in the new blocks they mine. Older transactions could not be changed. The chief defense against this in BTC is cost. Unless one hacks into 3-5 of the largest mining pools at the same time and controls them long enough to write fraud blocks of value to them, the last estimate of cost I've seen is above $7billion dollars worth of miners plus the operating costs of them. Next it would ruin the value of the coins they are steeling. I'd say, while technically possible, it's practically impossible now that the network is so large and robust.
A hard fork on the other hand can be done by just one person and cheaply. However, you have to sell that idea to the world and hope people (and miners) follow you so much that the idea of what Bitcoin is changes. Good luck with that. Miners have to think your new idea of increasing the cap will make them more money. Developers, users and node operators have to operate and build upon the new network you just created. Gaining a cap increase would be a very hard sell. None of the hard forks of BTC to date have even tried to change that rule and there have been many. Most go to zero quickly. Few have held on, BCH, bitcoin gold, bitcoin SV come to mind, but they kept the cap.
For my FIL, I would try to ask how he thought this would be done so he could discover, as we discussed the scenarios, that he didn't know what he was talking about. All in a nice way of course. My dad for instance trusts me to understand it for him because he knows I'm not pulling what I have taught him out of my hat. Thus, when the explanations get too deep for him, he believes me when I say, "trust me, it ain't happening." Incidentally, that's why I've learned what I know about this. Telling this to strangers that are just obstinate have made me a bit cynical and eventually I just give them the 'have fun staying poor' line and move on.
With my dad part of my 'trust' defense is, "Look Dad, this is where my money is. Your money is my inheritance. I don't wanna screw that up either. Plus, you're my dad. If I'm wrong, I bare the cost by having to take care of your needs if you go broke. I'm that confident."