How to fight landlord who won’t accept early release ?

[landlordhell]

Hi, my son and friends rent a house and have done for the last two years. One of the friends has bought a property of his own so is moving out and a replacement tenant is being sought. The landlord says he will only amend the agreement this once . My son and friends were all hoping to move on next spring( landlord is not good) but landlord is saying he won’t amend the tenancy more than once so they have to stay another 2 years. Do they have any recourse? Thanks

Before anyone asks,this agreement was done and dusted without involving us. The first rental they had, we were guarantors and checked everything over . Was all done via an agent but this was done differently so we’ve not been comfortable with it since we found out. Have passed all advice on but am not hearing anything back at the moment. So thanks for all of your thoughts .