Look at the DNS servers, those are almost certainly provided by the host.

That's simply not true. DNS is very often provided by the registrar or the host's host (or the host's host's host, etc.) in the case of resellers. And it could also be provided by a third party contracted by the host.

-sauoq
"My two cents aren't worth a dime.";

Ok, then "almost certainly" is over-stated. But it's certainly one of the easiest places to look and especially for the the larger ISPs (which are statistically more likely to be the host) it would work.

It is easy to look at but it's likely not to tell you anything or to send you off on a wild goose chase.

Most registrars provide DNS service. If the guy's client decided to use his registrar's services, the ISP (big or small) would be irrelevant. If you went looking for perlmonk's host and you found its name servers were:

Name Server:NS2.DEVELOOPER.COM
Name Server:NS1.US.BITNAMES.COM
Name Server:NS2.US.BITNAMES.COM
Name Server:NS1.EU.BITNAMES.COM
[download]

what would be your next step?

-sauoq
"My two cents aren't worth a dime.";