Red flag for me here with his or her expectations. It's hard enough to get PI attorney to rank in one city, let alone many - at least in competitive areas. If he doesn't respect what you have to say on the matter and insists on doing it his way, walk.
On the hand, if he/she is willing to listen, I'd recommend paid ads in either or all Findlaw, Lawinfo.com, lawyers.com, AVVO and adwords possibly, though I know one PI attorney who is targeting all of California and is paying $50K a month in adwords. It's not cheap. Ads in legal sites are more affordable.
There are also a number of sites that sell leads to PI attorneys. Those can be effective.
No matter which way you look at it, you have to pay to play. I say put your money where it's going to work for you. SEO in cities where you do not have an office, especially for PI attorneys in competitive areas, is likely a waste of money.