The real question is, how do you prove that their bitcoins were in fact lost?
If it's a simple case of lost private key, you can't. Why does that matter?
There are cases of outputs scripts that are provably unspendable. In those cases you can count those bitcoins that are permanently unspendable.
It matters because people lie, cheat and steal and should not be rewarded for it.
If the alleged loss can be absolutely proven to be unspendable, then I have no issues.