Thats why i suggested to him to show up early, to practise before the event starts. If it's well enough lit up he can manage with the gear he's got because i doubt he will buy a $1,700 lens for that event.
Of course it would be ideal to have a f/2.8 or better lens, but those cost a alot and since he has a D40 with a kit lens, i assumed he was on a budget.
First, showing up days in advance to check the lighting is a good idea, because if the lighting is poor and his gear can't compensate, then he's got time to make arrangements. If he shows up an hour early and the lighting is poor, no amount of "trial and error" is going to fix that his gear isn't set up for indoor low-light events.
Second, never did I mention or suggest buying the lens. I recommended renting it, as you can get them for very reasonable rates, usually $30-$50/day depending on how exotic the lens is.
The D40 has decent high ISO capability and a hockey stadium is usually lit well enough to get usable shutter speeds for non sports at f/3.5
Usually, possibly, maybe. That leaves a margin for error that I'm not a fan of accepting. If I make a commitment to get the shot, I make damned certain I've got everything I need to. The only reason I'm being so hard about this is that he's got precisely one chance to get the shot he needs to take. If you're out shooting for fun, you can use whatever. If you've got a specific capture you need to make, you need to make sure you can do it. If I were in his position, I'd probably rent the gear if for no other reason than to give myself that margin of error. PPPPPP.
Here's an example, (crappy subject i know) this is take inside a horse riding school, with seriously crappy lighting and it's shot with a D70 @ f/3.5 1/100sec @ ISO 400.....
As you can see, it's not too dark, or blurry, with a shutter speed of 1/100 at only ISO 400, i could have gone higher ISO but 800+ ISO on a D70 is less then desireable. The high ISO on a D40 is better i believe.
First: Even at that resolution I can see slight motion blur from the horses and people.
Second: You shot that at a wide angle, presumably 18mm. That minimizes the appearance of motion blur and gives you the largest aperture the lens offers, f/3.5.
Third: At a grad ceremony, he's not going to be shooting the grad portion at 18mm, more like 40-55mm, where the aperture is a small f/5.6, so there goes 2 full stops of light. 1/125 now becomes 1/30/s, and now both motion blur and camera shake are a problem. You're both shooting slower than the inverse focal length rule (by a factor of 2) and since the subject fills the frame, all motion is magnified.
If he wants to avoid using flash, he needs to have fast shutter speeds, end of story.