I had interest only loan on my rental property that I purchased back in 2007, with an intention of negative gearing benefits. It cost me $310k back then. Then I started paying interest + principal back in 2015. If I sell this house today, the market value is $650k.
So I don't think your friends will have a problem when they are paying interest + principal. Most probably they are on negative gearing, it means, they are paying less income tax. Less income tax = more money in their pockets. Worst come to worst they just need to sell the house & pocket the capital growth profit like me.
