Weird things can happen with animals. I can't speak from experience for larger herd horse breeding operations, but I know that beef cattle farmers typically expect an average
10% loss of calves per year, for one reason or another. Ten calvings in a year, expect to lose at least one. 100 calvings? Expect to lose at least 10. I'm not a horse breeder, although now I wonder if there isn't a similar "rule" for foals? Sometimes it's because the cow kicks/steps on/lies down on the calf. We had a cow lay down on top of her newborn calf one year and crush it, and they were in a 20+ acre pasture with plenty of room to maneuver.
Your friend's string of losses could well be bad luck, bad aim, bad timing, and accidents. The best of us have those kind of problems from time to time in a variety of contexts. Could it be something else? Possibly. It's hard to say, and harder to judge.