Java got uninstalled, Java is no longer associated with ".jar" and/or ".jnlp" files, or Java is just broken (happens).
1. When it asks what application to use, tell it to use a specific program, and browse to find "javaws.exe" found at (for Windows) "Java\jre7\bin\". Drive and installed location will vary, most people would have it in "C:\Program Files" or "C:\Program Files(x86)"
2. If a Java installation isn't found, make sure to reinstall Java
www.java.com. There's a simple app there you can use to test to see if Java is properly installed.
3. It's possible he turned off Java in the web browser. It's been known to have security issues. Simply open up Java's control panel and turn the option back on. (Security tab, "Enable Java content in the browser")
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