How can we check if this still exists / is it real?
You'll need to find some more information, the name of a Bitcoin exchange, some e-mail messages from a Bitcoin exchange, some written notes.
A bitcoin address usually looks like this
A bitcoin transaction id might look like this
You do not have to keep a Bitcoin address secret. You do not have to keep a transaction ID secret. Whatever it is vital that you keep passwords, seed phrases, or recovery phrases a secret from everyone, including the helpers.
If you have Bitcoin addresses or Bitcoin transaction IDs, you can check for unissued funds by entering these addresses or transaction IDs in a public Blockchain Explorer
Does he have a customer number?
He might have had a customer number if he'd given his bitcoins to a business that cared for him. You would need to find some of his emails or written notes that contain the exact name of the business he uses. There are thousands of them.
If he had his Bitcoin in his own wallet, he would not have a customer number. He would rather 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 may have written down a purse recovery phrase – a set of 12, 18, or 24 dictionary words that can be used to recreate a Bitcoin purse.
Do not give their passwords or recovery phrases to a helper. Do not enter it into a website. Each helper should just tell you how to use them, but they should not be watching or participating. Do not give anyone access to your father-in-law's computers, phones, e-mail accounts, etc. You are likely to be contacted by a large number of very honest scammers who are stealing you and charging / releasing fees and other fees will charge you some kind of fee you can imagine.