This is a common practice and you’ll find some other threads about it on AAM, querying who should pay the bill etc.
On the cost itself, €263 for 26 days at €0.30/kWh would translate to a constant draw of 1.4kW. Our heatpump is using about 0.7kW 24x7 at the moment for a 400sqm A2 house. So that bill is very high, probably because the heatpump was being run flat out, maybe to help dry the apartment (again common practice). It will come down significantly once they move in and set it to a moderate temperature I would say, I wouldn’t stress about it.