If you break down or have an accident on the road in Spain, then you must deploy two warning triangles to inform traffic that there is a possible obstruction ahead.
Also, any occupant stepping out of the vehicle must be wearing a high visibility yellow jacket.
Hire car firms are notorious for not supplying the triangles and High-Vis yellow jackets. You need two triangles and enough jackets to cover all the passengers in the hire vehicle.
If your friends hadn't deployed the warning triangles, and were just sat in the car waiting for the hire car company to turn up, then the Guardia Civil and Local Police probably won't stop as they haven't seen any warning triangles to signify that the car has either broken down or been involved in an accident.
Having said that. if they hadn't deployed the warning triangles and the Local Police or Guardia Civil had stopped, they could have been fined on the spot!