The first thing you have to look at is he investing for long term gains or is he trying for large amount of profit quickly. If he is buying and holding large cap stocks, highly rated bonds, etc, that is pretty safe, but those kind of things move slowly. Even if he is good at it he would likely be making 10-15% on his money per year. If he could make twenty he should stop running and manage a hedge fund.
Trying to make larger amounts of money faster means taking larger positions in riskier stocks. If you invest in a small company, say a biotech company, and they have one product come out that is huge in their field the value of that stock is going to skyrocket. But by the same token, if you take a large position and the company ends up second to market with their product, or six months later it turns out the product causes chronic constipation and suddenly no one is buying it then the stock is suddenly worth pennies on the dollar. Throw in the fact that corporate raiders in shadowrun use assault rifles and things can get really, really volatile. Nobody picks right all the time because there are unknown and unpredictable factors at work.
Also, there is the constant risk that investments under a fake SIN will be seized if the SIN is cracked. To avoid this he is going to have to be frequently moving money to multiple new and expensive SINs, or using black market services that will take a piece of the action as well. Either way it cuts into the profits.
Is he day trading? If so then he basically doesn't have time to be a runner. Those guys work 60-80 hours a week with extreme levels of stress an most of them loose money in the end.
So I guess what I am saying is that I would allow him to make a reasonable return on money he invests. Don't let him print money up from nothing, however, it just doesn't work that way. No matter how smart the guy is a part time investor with no staff picking his own stocks off of publicly available research isn't going to have money falling from the sky. He can either play it safe, and maybe double his money every five years as long as there isn't a recession, or play it risky, but then there will be times when he has big scores and times when he looses his ass.