All visa holders have strict restrictions when it comes to what they can and cannot do while they are under that particular status. Most of the restrictions that bestow an F-1 holder is centered on employment. The F-1 status is indicated to help foreign students obtain a U.S. education and you are not permitted to work at an outside job unless you have permission through OPT (Optical Practical Training).
However, your question centers on investment rather than working at the company. In that case, an F-1 visa hold is permitted to invest in a company and receive dividends. You have to be careful not to fall into the “doing business” category as that is not allowed based on U.S. immigration law and the F-1 visa holder still must file income taxes on the dividends received. Absolutely any involvement of business will require additional visas. It can be a bit of a gray area.
To make sure that you are following the letter of the law, you should consult an immigration attorney. One small misstep could lead to possible deportation for your friends.
can connect you with a skilled immigration attorney who can answer all of your questions surrounding F-1 visas and counsel you about your best plan of action. This area of law can be incredibly complicated so you should certainly contact a professional to guide you in the appropriate direction.