I suspect that countries like El Salvador might accept Bitcoin for such proof. How is the situation in your own country? My country will not accept Bitcoin because the government is anti-crypto.
If you even show this in your country where they’re strictly against crypto, they can even get you arrested as to why you’re holding crypto when it is highly prohibited by all citizens to own and trade with them. I won’t even think of doing that when I know my government is anti-crypto, it’ll make me more vulnerable to the public (government agencies) if I do that.
Nope, even stock, gold, real estate didn't qualify as proof of funds, so I'm not surprising to see Bitcoin didn't qualify too. However, I'm shocked that your friends didn't have emergency funds, no matter how big your trust in investment, you need emergency funds in order to prevent you to sell your investment at loss.
Bitcoin to some people is an asset just like gold, real estate etc, so it can’t be used as a proof of funds when applying for visa. Where bitcoin is legally used and a means of payment and services, I think such countries will accept it as a proof of fund. In this case, we can’t tell if the friend may have emergency funds with him but it might not be enough to show as a proof of funds that will make him qualify for the visa. What was he thinking if he doesn’t have fiat to spend in the country he’s going to, he will definitely have to convert there if he doesn’t now, so I feel he has some funds but not enough to allow him qualify for the visa.