Small capacity cylinders fall outside the testing regime applied to dive bottles (2.5 yrs) and surface use bottles (5 yrs).
Out of date cylinders for PCP weapons will only be problematical for National and International events run under ISSF rules.
If your friend is still concerned, the cylinders can be resealed by a competent person or gunsmith.
My Pardini K12 has cylinders dated 2015 and has been resealed and checked; I'll get them done again next year.....
ATB
Mike.