iMac 2007 / 2008 - can't login after update, please help!

Hi there,

First off, sorry for the lack of specific details but I'll give as much as I know!

My brother has been given a 2007 / 2008 iMac (one with a black back) and he can't log in since an OS update. He doesn't know what OS he was trying to update from unfortunately, but he tells me he was trying to update to "the latest one" in March so we should assume he's tried installing Yosemite, although the background image at log in is still the space / stars image so not sure if this is the case.

He chooses a profile, enters his password and then the screen goes briefly blue before just going back to the log in screen. If he selects 'guest' profile the box just shakes and does not go further, so he's stuck in a permenant loop. Have tried booting in safe mode but even when holding down cmd and S the iMac just boots normally and he ends up at log in screen again.

Any ideas on what he can try to get past this?

Many thanks