District heating or another heating technology?

At 13,500 kWh per year, that would be about €400 additional costs.

Moreover, with KFW 55 and 160 sqm heated area, he would need about 7,000 kWh.

Does the contract say anything about how much he is allowed to raise the prices? If next year the 8.33 cents become 15 cents, that will of course be tough...

I would pay close attention to that too. Presumably, the price is linked to some index.

However, I think there must also be consumers who have no problems with the rising prices.

There are streets here that pay absolutely nothing for district heating consumption. These are contracts from the early days when people still thought district heating was the thing of the future and only needed a "few" people at the start to make the whole thing profitable and thus attractive. This regularly comes up in the local press during winter because the supplier no longer likes the contract so much anymore ;-)