In my social circles when I was growing up, it was reasonably common for people who knew a teen well (family friends, uncles/aunties etc.) to slip them a few quid to say well done when they got their exam results.
I'm now living in a different part of the country and it's quite a few years later, and I don't know if that's normal any more — but also, I'd like to give my friend's kid £20 or so right after exams finish instead, to send the message that it's a "well done for trying your best and getting through them" rather than about what results he gets.
I'd check with my friend first too to make sure it's okay with her, obviously, but if this is something that people just don't do any more or never do before results, I don't want to make her uncomfortable by asking. Would you think it was weird if your friend gave your teen money to say well done for getting through GCSEs? Or should I wait for results? Or was this something particular to my area as a kid, and something people generally don't do at all?