Unpaid train fines in Netherlands (and how they apply to non-eu citizens)

I can't help but wonder if OP and his friend aren't actually the same person...

However, If he really wanted to visit Netherlands again (in the future), couldn't he just enter Shengen at, let's say Brussels airport, go to Netherlands by land, and come back to Brussels, and exit Shengen at Brussels again. Since, in this scenario he'd never deal with Netherlands control, he also wouldn't have a problem.

Sure sounds like a nice plan to save 40 €. Might lose many times that much on flights and travel though. Avoid the possible hazzle & pay up.

I bought a train ticket in Netherlands in 2009 with a discount card but it was already expired. The conductor checked my card and said that I was supposed to pay the fine 35€ and also the full fare 28 €. I made a decision not to pay because I was going to leave Netherlands in a few weeks. I live in CZ and I was afraid that Dutch authorities would ask Czech authorities to execute the money but nothing ever happened. I talked to my Dutch friends and they told me that if I don't come back to Netherlands I am safe, but if I ever decide to go back and the police checks my passport I might be forced to pay immediately. I would like to know how long they are going to keep the fine in their records, I hope that after some years it will be lapsed.

I would like to know how long they are going to keep the fine in their records
#22. You can write to NS and I´m sure they´d reply. They are the only people who will have the answer and they would be pleased to hear from you.

The conductor checked my card and said that I was supposed to pay the fine 35€ and also the full fare 28 €
You were riding the train without a valid ticket, and you found out the hard way NS deals with passengers like that. Bad luck, but that can happen when you don't pay for a service you use. I think I mentioned this before, but if you had an excuse for not paying the correct fare (e.g. you overlooked the fact that your pass expired just the previous day), you could have tried to ask NS if they could pay back (part of) the fine. They are actually quite lenient towards people who incidentally board without a (valid) ticket.

I made a decision not to pay because I was going to leave Netherlands in a few weeks
That's fine, but making a decision while knowing it may backfire sometime in the future means accepting the consequences should you ever get in trouble. Unless so find a user on this forum who's an expert on this matter who can give you a signed letter stating that the registration of your fine has "disappeared", no one here can give you 100% guarantee that you'll be safe. That said, regular border controls between CZ, Germany and NL have been abolished and Dutch police is only allowed to perform a random check if they have serious grounds for suspicion, so chances are no-one will find out unless you get stopped by the police for another violation.

I hope that after some years it will be lapsed
I hope not, frankly speaking; I've always paid for my ticket and get sick of all those willingly abusing the system (that includes of course Dutch passengers who board without a ticket or go to 1st class after paying for 2nd class). In fact, I truly hope NS will crack down even harder on passengers riding without a valid ticket, make them pay on the spot and otherwise handing them over to the railway police.
But I guess that won't happen, and NS will just raise its fares again soon, partly basing it on the number of people riding without a ticket.