This question is rather US-based but perspectives from all other countries (UK, Aus, CAN, etc.) are much appreciated.
The situation is as follows. An expat gives some money away to a friend in his home country, which she uses to do the business. He doesn't have the custody, ownership, or any other legal association with such money. For example, his friend may buy shares or register a company without him as an owner. A decade pasts and her business goes well. At the same time, assume our expat fella doesn't have a good run and he's now unemployed. Luckily, his friend becomes an oligarch, so she sends him a large sum.
The question is: What is the expat's taxable duty on the money he receives in the country he's living, say the US? In particular, is tax only applicable on the money he gets? Or, is he also somehow responsible for the, say, capital gains, of the friend's business? Does the answer change based on whether or not: (1) It's a personal loan instead of a giveaway? (2) He's aware of the friend's business or consults her, such as which stock to buy or operational strategies of the company, even though he still doesn't have legal associations with her business as mentioned above?
EDIT to add further clarification based on comments: I'm interested in knowing in particular if the expat is subject to any tax considerations from what his friend does with the money? For example, the friend makes 1k in capital gain in the first year from investing activities, does that impose any tax on the expat? Again, the money is legally no longer his money even though he may be aware of what the friend does (like, they talk about what to buy or sell or he may even give her advice).