I agree because it is extremely risky if he use the company funds that are needed in operation and other aspects of running the company t invest in. Even if it is in good intention,if he loses money then his company will halt its operation and other developmental aspects that should have been made with the money.
You are right, but even if the money isn't intended to be used soon for company operations, it is still not advisable to give such suggestions because it is his company and he should decide what is good and what is bad for it so that the good or bad consequences aren't put on someone else.
If he invests the money after the suggestion from his friend and can't manage the investment in a good way and loses it or even a portion of it, the friend is going to be blamed for the loss which isn't what one would want to get after giving a genuine and selfless suggestion.
This is why I would never do such a thing, even if it's a very close friend, I would only tell him to try and find some ways to improve the company or increase the funds but wouldn't suggest something specific.