Can the victim of a 419 scam sue the email provider if the provider failed to take action after they were notified of the scam?

I would have to say yes. But it depends weather you ask them or not. If you ask them and they say yes, give them a little bit, and is they never do, you probably can. But I wouldn't recommend it with higher up e-mail servers like Yahoo(www.yahoo.com), G-Mail(www.google.com and type in 'g-mail'", etc.

No. For the most part, federal law protects internet services providers from liability arising from the actions of its users. If an AOL user publishes a defamatory message, AOL is not responsible for that person's actions.

The best option is to file a complain with the Internet Crime Complaint Center (IC3)

A partnership between the Federal Bureau of Investigation (FBI) and the National White Collar Crime Center (NW3C). Alerts for consumers and easy-to-use ...IC3 accepts online Internet crime complaints from either the person who believes they were defrauded or from a third party to the complainant. We can best process your complaint if we receive accurate and complete information from you.

http://www.ic3.gov/