I'm sorry, but anyone who has inappropriate pics on their phone knows they are there.
In fact, I personally believe some people are capable of sexually abusing children to get back at the Mother. "Oh, it was an accident" is a thin excuse for something that could be seen as grooming children and desensitising them to sexualised images and behaviour.
I'm sorry. But I find minimising and saying "oh maybe it was an accident" is just another form of codependent enabling.
If this was a neighbour alone with your children would you accept this behaviour?
People know when they have their sleazy pics on their phone. Children grow up into adults, and they ask questions like, "why didn't you protect me?"
If your Mother won't protect you, be your last bastion against all things, then who will be?