In reply to get to the punchline:
Bad luck. Certainly dehydration will contribute and any period of inactivity over four hours is a risk . Solution : drink lots pre travel and this means your always stopping for a pee, so on you legs for a couple of minutes , which is all you need to use the leg muscles to promote blood flow and stop clots forming .
Of note , it would be common practice to place someone on anticoagulants for 3-6 months after a PE, so if your partner is on these , make sure you do not have any sort of bang to the head , worth wearing a helmet even down the wall etc .
Hope all well.