Having spent the better part of two years trolling that pond,I would recommend that he do the following:
1. Join all the groups he can that relate to what he wants to do. This will make sure he is in the know, will allow him opportunities to socialize with his network, and (most importantly) allow him to connect to random strangers. Once you are in a group with someone, you can connect to them directly without an email address, work history, etc.
2. Use a generic connection greeting, unless you have something specific for that person. If you met Bob at a convention, then you can put that in your LinkedIn connection invite. If you have never met the person, use a general "I am looking to connect with other professionals in this field." If you put a lot of information in the invite, it will either scare them off, or be lost once they accept your invite. Once they have accepted, Move on to step 3.
3. Contact them at the email they list on their LinkedIn profile. Most people provide an actual email address on their LinkedIn account that is visible once you have connected. Follow up with them at that address, instead of inside LinkedIn. Start the email with "Thanks for connecting with me on LinkedIn, I am...."
4. Purchase the thick skin upgrade. People will not connect, they will not respond, they will connect, but take years to do so, or they will connect, and then not respond. It happens. But if you are putting enough feelers out there, you will get some bites. And eventually those bites will lead to jobs.
The best feeling I had was sitting down with a VP of a company, mentioning people I was talking to via LinkedIn, and have them know those people, and even know their bosses. That's when you are starting to get somewhere.