Emma, there's a positive spin on this.
People with ptsd often have a lot of difficulty knowing which emotion is 'right' to have- hard to tell when you have 30 in an hour ( :( ), so they look to 'cues' of other's emotions or actions/ reactions, and then reason from there.
(And for a person with ptsd, the 'blame' or reason will usually come back to them, in their mind, something 'wrong' with them or something they must have 'done' wrong).
So, if you say nothing at all, the person often thinks they've made a mistake- obviously there is no feedback- that must be 'negative', and (thus) their way of avoiding you/ it. -Burdensome.
If you however acknowledge it- what you feel about it (whatever that is- shock, anger, horrified, sad - as to 'what' occurred, what 'you' feel), but have the attitude "Great! Think that's so great you told me, you're doing better!", and then carry on- have some laughs, that will help later.
For example, you could say, "I'm sorry that ('x'), (or) I can't imagine how you feel but I would feel ('x'), but I'm sure glad you told me!" :)
A member on here gave an example, she (felt she had) an extreme reaction at her therapy, and her T said just that- all smiles/ happy, basically- "Great! That's progress!", so the member said although she still felt like 'crap' she had enough trust in her T's 'assessment' of it, to then go home, not blame herself for causing 'harm', or wondering what her T thought of it, and hoping that she would understand it more later (and hopefully also see it in a positive light).
I can only say myself I feel ptsd is something to 'deal with' alone, hard to explain (it) and you don't understand it all yourself, and because of how it is don't want to 'expose' it (or yourself) to others. So (that's) difficult.
See what works. Try different things- everyone is different.
Try to have positive times. Even people with ptsd just want relief- not 'heavy' all the time, distractions are better. Especially when you're stuck with this 24/7.
ButterflyPEI wrote a good thread called "A Letter For My Family and Friends" that expresses it far better than I can.
xox