That is so difficult. I think it is typical of the survivor(s) of traumatic death to wonder why wasn't it me that died.

No matter what the skill level of equestrians, horses are 1000+ pound animals with minds of their own and even a calm horse can freak out when they perceive danger. That is just their nature because they are prey animals.

Your friend keeps replaying "what if" & "I should have" which is very common after witnessing such a horror.

It doesn't sound like the right time for your friend to understand but anytime people choose a risky endeavor like rock climbing or skiing or parachuting or even riding a horse you accept that it may not end well. That is why I no longer ride but take my horse for hikes on his lead rope.

It is going to take her quite a bit of time to recover and even then her life will be changed forever. It has taken me ten years and much therapy to find peace with my own PTSD. I now accept what happened but will never really understand why it happened.

Gently encourage her to seek therapy but if she won't all you can do is listen and encourage her.