Depending on his title on his website, a pr7 link, or even 10 of them may not be enough to make the jump to pr6 on his site.
Without a good, deep analysis of his comp. I would not advise buying links, I would concentrate on just making sure that he gets links from sites with his keywords in the title and content.
He can have a link from a pr9 site and his link on a
pr 3 page and the link is credited as a pr3. The index
pr does not count unless the link is on that page, Link value is calculated from the PAGE the link is on, not the site, If it were counted from the site, all pages would have the same
pr on all sites. and to get a
pr 7 page link with keywords the same as his is going to cost more than it is worth.
The
pr 7 guy knows he will be bleeding
pr to a pr5 site.
I would be more concerned with quality of relevance, not
pr of the site.
Link buying is something that is risky at best!