their responsibility is to get them to their destination and look at other airlines - not just KLM or even just skyteam. there are no seats on KLM from ATH to LBA today so has your friend been confirmed?
the only thing that doesn't apply for weather (usually) is compensation. all other entitlements to rebooking and duty of care apply regardless of the reason for the delay/cancellation.
i hope your friend has promoted them to look at all UK airports and not just LBA. i don't believe for one second there have been no seats over the last 4 days to MAN/LBA/LHR/LGW/NCL etc. on BA/A3/LH/LX/EI/AF/SN/IB/U2 etc. at the end of the day i would have taken something to anywhere in England or GLA/EDI and got the train home - the train fare would be claimable from KLM. your friend should push them to look at these other options - and if they refuse it will feed in to the compensation issue i note below.
KLM are also responsible for expenses (hotel & meals) whilst your friend is stuck in ATH so they should claim that all back from KLM.
normally compensation would not apply for weather. but if the airline doesn't actually take opportunities to rebook them on other airlines to get them home days earlier, it does become payable. not something to worry about today, but it is something to look at when they are back.