If she has two mortgages on the same property then the one taken out second is most definitely a top up and UB are correct, she cannot pay off the original mortgage first and leave the top up continuing. The reason is that top ups were done in a different way and did not normally go through a solicitor so the original mortgage is the one registered. They may have separate account numbers, it doesn't matter if they are both on the same property.
What she should do to get around this is pay off a very large lump sum off the original loan, whatever amount is required to leave her monthly payments at say €10 or even less, leaving the term the same which is probably the same as the term on the top up. This way she will benefit from getting rid of the bulk of the higher rate loan and can continue to pay the top up.
The bank are not playing games on this and whoever informed her originally that she could clear it and leave the top up was just incorrect, it is an uncommon enough scenario so might be hard enough to find correct info for them especially on a First Active loan.