Damn, once it clicks it's hard to unsee it.

Heroin might be healthier.

Just for my curiosity did you have any luck? That long string you mentioned might be the public address. If it is you can look up what's in the wallet to see if it's worth your time to mess with.

Well what's the service? Did he buy bitcoin through a service of some sort, or actually set up a wallet? You're in a hard spot of needing to share more info to get a better answer, but sharing the wrong info can lose it. My best suggestion would be read more. Are there any headers on the paperwork with a company name?

If you don't have the seed phrase it's not necessarily lost. If he did buy the bitcoin himself and transferred it into his own wallet it could be on an old flash drive, old computer etc.

There are I think 2048 words used as seed phrases. It might be worth looking up that list of words and seeing if any of your paperwork looks like them. If I remember right you only need the first 3 or 4 letters of each work to have the whole word since it's a limited list of words they can be if that makes sense.