There is no PSU wattage requirement info for the ASUS P8Z77-V board on the ASUS website. Loads of specs and features but nothing about what PSU is required.
A Google search didn't bring up that information either, so all I know is you need an ATX-type PSU, but I guess you already knew that.
A 620-watt PSU should be okay since that board is known to consume less power than most, but that's only a guess.
he has got a 690 after all.. Nvidia say minimum 650W but since it's close to 650 and a good make you should be ok. since the card maximum power consumption is 300W i doubt the rest will take 300W.
Yeah, it'll be OK realhardtech suggests 620w (min 46a on the +12v), M12 has 48a on the +12v
and they are thorough n accurate on these recommendations.
So, that Seasonic should be fine even with liquid cooling.
Wouldn't hurt if he went a little higher (n got 50+a on the +12v)