Can anyone help out a friend regarding diving near Puerto Vallarta?

[vnwelcome]

A good friend is visiting PV in a few months and I told him I'd check here for diving info. I've searched the archives and gained a little info. I picked up on a few dive op names but none really seem to jump out over others. Maybe someone can offer insight into the following:

When's the best time of year to dive there and why (he's a repeat visitor there)?
Is there a particular area or site that stands out?
Is the diving there beginner friendly and if not, why?
Are there overnight or multi-day trips that offer opportunity for better diving?
Any other general recommendations for visiting the area?

Thanks for any info you can offer.