Is it illegal to make a trading card game where the cards are real money? Asking for a friend?

Yes. Only the government can produce currency.

You'd have to check the laws of your country, most likely the gambling laws

For example, in Japan most forms of gambling is illegal. There have been some loop holes such as you earn tokens which have no value to them, coincidentally there is a store next to the place that will accept those tokens and give you something in return. These stores are entirely seperate from each other which makes it legal

i wouldnt think so