qibla

Facing the wrong direction when performing Salaah

Q: A family moves into an apartment, they use a well known Qibla finding app to locate the Qibla direction and pray. Months later a neighbour points out that the direction is wrong and they must face the other direction. The difference is about 90 degrees apart. What happens to those Salaah that were prayed in the incorrect direction, are they valid or do they need to be repeated?