As Newby points out, the details are very hazy.
If they moved out of the apartment and sold it 8 months later, as seems to be the case, it was the PPR until they moved out and they managed to sell within the 12 month exemption period. If this is the case, no CGT due.
They will have additional headaches (possibly, depending on what actions they took during the whole process) such as claiming TRS relief on the "new house" when it wasn't a PPR, possible CGT (and SD?) implications on the "new house" depending on what happened in the two year period, etc.
I'd certainly echo the suggestion that they get a tax expert to review the situation. There are too many elements to it to get/give meaningfull advice without the exact details. I'd suggest not getting overly concerned about the potential interest rate until they figure out if they actually do have a liability (they may already be 100% certain they do, but it's not clear from the details given in the OP).