At 12 weeks they are still quite diddly & can easily get caught out. I would keep in one room with a tray of worlds best and a tray of fine clay litter like sophisticat choice, tesco sell it.
If you leave a poo in the tray & show them its there, it may help them click onto the right place to go.
If they were never litter trained in the first place then they may need caging to break the habit.
I would recommend your friend to clean the spots they go with a pet odour remover such as simple solution (all pet shops stock this)
Out of interest are your friends trays lidded?