[Cry Me A River]

[ QUOTE ]
Hiro can't change the past,

[/ QUOTE ]

Except he did change the past.

There's a photo of Hiro with the waitress, taken six months ago, that would not exist without Hiro's time travel powers.

That's not changing the past in a big way, but it is changing the past.

In addition, if Hiro can't change the past, why did future-Hiro travel into the past to talk to Peter on the subway?

Hiro's wrong, he's just frustrated by a coincidence - That the waitress also had a brain annurism as well as his lack of mastery over his powers (the accidental time jump).