Remove old cable connection?

Is there a way to dismantle a cable connection that has been inactive for ages so that it can no longer be used?

Pretty sure that’s possible. You call the cable network operator and tell them you want to dismantle the connection. Then you fight your way through the hotline and eventually a crew will come to remove it at the distribution box, if possible. The bill will definitely be sent to the property owner. Honestly... paying unnecessary cable fees for a few months is definitely cheaper than a dismantling.
And if your friend is the really smart type who thinks he can dismantle the cable connection first and then submit a special termination a week later due to lack of availability, the lawyers might be happy.

Some seem to believe that they are the one-eyed man in the land of the blind. The cable company weiss that there is cable there. Now you want to tear down the box, call and say "Yes hello, there is no cable here, someone tore down the box, I want to terminate the contract prematurely!!"

And then? Then the company will install a new box there and you will have cable again.

Vodafone also offers fiber optic. Just apply for the switch to a fiber optic tariff and then you can convert the contract. Sorry, your "friend".