Is there a regulation that defines when to use QFE or QNH for landing?
AFAIK there is no EASA regulations forcing a pilot to use one or the other. The default option is to use QNH, which shall be transmitted to all aircraft:

(3) Except  when  it  is  known  that  the  aircraft  has  already  received  the  information  in  a directed transmission, an QNH altimeter setting shall be included in:
(i) the descent clearance, when first cleared to an altitude below the transition level;
(ii) the approach clearance or the clearance to enter the traffic circuit;
(iii) the taxi clearance for departing aircraft.

But there is an option to use QFE as well, and it is commonly used in some places, for example for certain operations in the UK and sometimes for military operations.

(4) A QFE altimeter setting shall be provided to aircraft on request or on a regular basis in accordance with local arrangements.

The &quot;in accordance with local arrangements&quot; part also incidates that EASA has left it up to the individual local authorities to decide whether or not to use QFE - so it is unlikely that EASA has a common rule for this.
Both quotes are from SERA.8015