I am an optometrist.
Yes, this is very, very common. Now, I think some of my colleagues would class this as "malingering" or "having us on" but IMO that isn't always the case. I should say that I'm also a behavioral optometrist so may take a slightly different view from some other optometrists.
So either your daughter is pretending or she's not. If she's pretending, that's caused by stress, if she's not pretending that's also very likely to be caused by stress. In my own office, I caught one little lady out when she said she couldn't see any big letters either close up or far away but then told me I'd spelt her name incorrectly on the PC! The letters on the PC were smaller than the other letters she said she couldn't see.
There is probably nothing wrong with her eyes themselves, they probably look healthy. But sometimes in a stressful situation the ability to focus or change focus can be interrupted. IMO this often seems to be the problem, although not having seen your dd, I can't give you a diagnosis of the problem, as such.
Now, if you wanted extra reassurance, you could bring her to an ophthalmologist but I would almost place a bet on it that they would find nothing wrong. In one case, where I referred a little girl, the ophthalmologist confirmed that there was nothing wrong, but still prescribed a weak pair of glasses as he felt that she needed attention and he was going to give her attention! Her mother went along with this and I think the little girl wore them for a few months and then forgot about them. In that case, I had a feeling there was a stressful home situation as well.
Brambles is also right, type 1 diabetes can cause blurred vision but given all she's been through, and if she's got no other symptoms, I would say it's unlikely.
And in my professional opinion, your X is a twunt! How can he think she wouldn't be stressed!