You have nothing to worry about yourself. Pepper can't change the terms and conditions. There is a small chance that they might offer you a discount to refinance the mortgage. But with only 12 years left, it is unlikely to be a big discount.
If your relative is paying his interest in full, Pepper will not be in touch with him, unless they want to offer him a deal to pay them off early.
At the end of the term, he will have to sell those properties to repay the loans.
If there is a shortfall, then he will still owe Pepper that money. If he does not pay, they could go to court to get a judgement order against him. If he still does not pay, they could register that as a judgement mortgage on his home. This means that if he sells his house, he would have to pay off the shortfall to Pepper first.
If he is on interest only on a cheap tracker, the rent must be greatly in excess of the interest he is paying. He should start using the surplus and any other savings to build up a fund to pay off the mortgage when the mortgage becomes due.
He could use the surplus to pay off the mortgage on his home. But I think he should organise his finances in such a way that he eliminates or reduces the negative equity on the investment properties.
He should have a chat with Pepper to see if they would offer a discount for early repayment of the investment properties.
Brendan