does it means those funds will be trapped in somewhere and no one cant use them ever again
For all intents and purposes... Yes.
The odds of someone 'randomly' generating the private key to that address is so small that you can effectively consider them lost forever. There is a chance that if quantum computing ever becomes reality that it might be possible to crack it, but I wouldn't hold my breath if I was you.
This scenario is precisely why HD wallets were developed... all addresses/keys can be recovered from the single seed... and generally, any backup will then be able to recreate any missing addresses/keys that were used after the backup was made.
Try downloading the electrum wallet, installing it, then importing your private key into it.
There is no point... the addresses containing the "Big Payments" were created AFTER the backup was made... so now that he has restored from the backup, those addresses are effectively lost...