I think ignoring this is the wrong response.
I think that you may be catching this right at the onset - until now there were only basketball games etc. It may be the first major fall, and what better time to teach someone that there is always an עין רואה.
Also, most that fall want help to get up again. It's not doing him a favor to let him stumble into a hole that he may never be able to climb out of.
Hashem arranged that you should see this. The onus is on you to help him.
How to do it? I don't know. It's something that requires remarkable tact, probably daas Torah. But simply to ignore taking steps that could save someone's spiritual life is potentially לא תעמוד על דם ריעך.