But she'd also have to stop to get to a restroom to put one in. Some people have heavy periods right at the start, so she probably couldn't even get there by the time she realized her period started. I for one use a pad for a few days before when I think mine is coming just so I'm prepared. Tampons are harder to do pre- flow.

Having pads/tampons in her purse or car doesn't help the fact that her pants are now imitating a crime scene. Irregular periods make it very difficult-to-impossible to even wear "just in case" pads. OP, I've been right where you are. I was mid-period, had a sneezing fit at work, and had to go home and change because my pad had been overwhelmed (hello, clot the size of my fist!). Female anatomy can be a real bitch some days.