<p>The first sentence requires nothing more than a comma before the who. The who phrase definitely refers to Jack but it needs a comma because it is a non-restrictive rather than a restrictive who phrase (meaning the who phrase is added to provide additional information and not to restrict the meaning of what becomes before the who; an example of a restrictive meaning would be adding the who phrase to designate which Jack you are talking about if there are two Jacks). The second sentence using “Jack’s” is wrong because “who” cannot be used to refer back to a possessive form of a noun, which the sentence is trying to do.</p>
<p>The following lists a number of idioms but I cannot say it has all (note you need to click on each letter in the alphabet box upper left to continue to another page): <a href=“http://www.learnenglishfeelgood.com/americanidioms/index.html[/url]”>http://www.learnenglishfeelgood.com/americanidioms/index.html</a></p>
<p>Just note the phrase you mention “as a means” is not an idiom. The word “means” is defined to mean a method of bringing about a result and is not actually a plural of the word mean which has an unrelated definition.</p>