I knew exactly who this would be without opening the link!
Yes its very foolish and she is setting her horse up to make a mistake, which could end up with him seriously injuring her or someone else.
I do not doubt she knows him very very well, and that she believes he wouldn't try to hurt her, however she is building a false sense of security here, forgetting in some ways he is an animal, and whilst he wouldn't intentionally want to hurt her, he has NO idea of how easily injured a human is.
It would take one slip, one misjudged rear, one 'I think she's playing but really she's trying to keep her balance in the mud and hasn't seen how close I am'.. for him to knock her flying, boot her in the head etc etc.
This is how humans end up dead - a friends daughter died as a result of a horse playing with her. She was walking away, he darted up and grabbed at her hat, a game they'd played often, only this time she had her hood up over her baseball cap.
He grabbed her by the hood/back of the jacket and instead of flinging a baseball cap into the air, he flung HER, and her neck broke, she was dead before she hit the ground.