Once again - discretion is given, not expected. Once discretion is expected, it changes into a rule, then discretion morphs itself into a new level (and we get new arguments).
There's not much more to be said on this thread unless I go black and blue. If I were to add another data point, MH were strict with my mother for being about 2 kg overweight when she checked in at KBR (KBR is of size to KUL as HBA is to SYD, just to give you an idea). So, she ended up repacking and once she was dead on the mark, they let her go.
I was also pinged by ZL at OAG for being similarly overweight by about 1 and a bit kg. I'd already repacked once, but they would not budge until I had "reduced the weight by a little bit more". I eventually did the "hidden weight" trick with my colleagues so I could check in my bag with only a few hundred grams overweight. (And it was almost for nought in the end when ZL forced us to volunteer luggage to be offloaded before we could board, since the flight was full and overloaded).
Also, as I mentioned, when you throw in the fact that you may be dealing with outsourcing companies at check-in (e.g. another airline's staff operating check-in on behalf of another (whether in the same alliance or not), or a generic ground handling company operating it), the audit marks easily left behind enforce staff to stick to baggage allowances to a tee (or engage in some "creative accounting"). I've seen this both abroad as well as in my home airport, BNE.
And, as most said here, your friend could've easily helped themselves. If the system wasn't going to be flexible then there was an easy way to avoid the excess charges. And giving the whole lousy spiel of, "Well I 'got away with this' last year, why not now" is frankly pathetic. In any case, your friend acquiesced and paid the charge, and assuming that your friend wasn't giving attitude as you so described, then even with their conclusion they told you then they must've been graceful enough to consent to payment instead of finding another solution or throwing a massive tantrum.
Perhaps your friend (and you) should consider flying EK next time. Their allowance in Y allows up to 30 kg of checked bags instead of 23 kg. My suggestion to you, however, is not to try checking in 31 kg without expecting resistance or a suitable plan B.