How can we check this still exist/that this is real?
You have to find some more information, the name of a Bitcoin exchange, some email messages from a Bitcoin exchange, some written notes.
A Bitcoin address usually looks like these examples
A Bitcoin transaction ID might look like
You don't need to keep a Bitcoin address secret. You don't need to keep a transaction-id secret. However it is vital that you keep any passwords, seed-phrases or recovery-phrases secret from everyone including helpers.
If you have some Bitcoin-addresses or Bitcoin transaction-IDs, you can check if there is still unspent money by typing those addresses or transaction-IDs into a public blockchain explorer
Does he have a customernumber
He might have had a customer number if he gave his Bitcoins to some business to look after for him. You would have to find some of his email or written notes that give the exact name of the business he used. There are thousands of them.
If he kept his Bitcoin in his own private wallet, he wouldn't have a customer number. He would more likely have one or more of
- a wallet program on a PC or Mac
- a Wallet app on a smartphone.
- a hardware wallet (looks a bit like a USB memory stick)
- a paper wallet (random looking numbers and letters)
He might have written down a wallet-recovery phrase - a set of 12, 18 or 24 dictionary words that can be used to recreate a Bitcoin wallet.
Don't give his passwords or recovery phrase to any helper. Don't type them into any website. Any helper should simply tell you how to use those but should not observe or participate in using those. Don't give anyone else access to your father in law's computers, phones, email accounts etc. You are likely to be contacted by a great number of very honest seeming tricksters who will steal from you and demand unlocking fees / release fees and any other kind of fee they can think of.