Is it okay to let someone else's apartment fall into disrepair?

Where do I even begin?

An acquaintance let his son and daughter-in-law live in his apartment rent-free. After 15 years, they finally moved out, leaving the place a complete wreck. They didn't lift a finger to maintain the walls, the woodwork, or anything else. It’s the exact same furniture he bought when they first moved in. It feels like they just used him for a place to stay while blowing their cash elsewhere.
The man is retired and doesn't have the funds to fix the damage and bring the unit back to decent condition.
What are his legal rights here?
Should he actually sue his own son?

In principle, there aren't any special legal rights at play here. I mean, do people honestly think a father would actually sue his own son over something like this? If someone can't even afford to flip an apartment, how are they supposed to manage a lawsuit when they clearly don't have the funds for a decent attorney? Please don't take this the wrong way, but I'm a bit confused by the situation you're describing. So, the son was living in his father's place with verbal permission, which isn't anything out of the ordinary, right? He didn't exactly wreck the place; he just didn't repaint the walls or touch up the window frames—which, unless there was a specific agreement to the contrary, is really the owner's responsibility anyway. Now, is the father seriously expecting someone else—in this case, his son—to step in and renovate his property until it looks brand new? 😂
Based on everything you've shared, it seems to me there is absolutely no way to legally force a son to do something like that.

He has zero rights here. If he’s just using the apartment as a temporary place to stay—and for free at that—he isn't obligated to sink any money into it unless he feels like it. His only real responsibility is to avoid causing damage, and based on the post, the place is being kept in the same condition it was originally in. I don't see an issue here. As for where the cash came from, that’s entirely up to his son.

Thanks for the input.
I honestly don't know what to say to that. In my view, if you use something, you maintain it and replace whatever is broken or worn out. That's just how I see it.
And that line about "leaving the apartment in the same condition it was in"? Ha! It sounds ridiculous to claim you've used furniture for 15 years and somehow leave it exactly as it was originally. :-)
If something isn't maintained, it is impossible to leave it in its original state.

"Fix his place up so it looks brand new?" I can only take that as sarcasm. Imagine furniture and woodwork that hasn't been cared for in all those years; to me, that's simply not decent or civilized behavior.

Things wear out with use. His father let him use it for free. Whether the son decides to replace it out of good will is entirely up to him. We can agree on the cultural aspect here. But you asked if the father has any actual rights. Saying "the apartment is maintained in its original condition" shouldn't be taken literally to mean he's looking for torn chairs or minor scuffs. If there’s significant damage, then you have a case. Best regards! 🙂