How do I get people to register and go to a live workshop?

Yes. For any of us to offer useful advice, we need to know quite a lot more;
Is your friend charging? How much?
What is the subject?
What are the attendees going to learn?
Is he an expert? Is he recognized as an expert?
Does your friend already have a following? How many of these workshops has he already put on?
Is he giving these workshops as part of an event, or are these stand alone events that he is putting on himself?
Is he going to travel around the country, or just stay in one spot?
What is he selling at the event?
How much time do you need to promote the event?
How much money can you spend to get each person to attend?

Putting butts in seats is an exact science. But for an example, in most marketing seminars, it takes a couple hundred dollars to get one person to attend. That includes list selection and repeated offers.

One solution is to have a few speakers pool their lists and share expenses and just book shows where they all speak in the same venue.

Another way is to simply use someone else's audience, and book your seminars into an existing event. That's mostly what I did. Book your show in an event where your ideal prospects are already planning to gather. Usually, you'll do these events for free, and make money on sales of books, DVDs, consulting.

In that case ($35) you need to book your friend into free speaking venues so he can offer his free event...to sell this paid event. In other words, NOT give his free event at the speech, but instead book attendees after his short free speech.

So it would be;
1) Give a short pre-view to sell the free event.
2) Give the free event to sell the paid event
3) Give the paid event.

For example, he speaks at a local Chamber Of commerce. He may have 20 minutes to give a short speech. This speech wets the appetite for the longer FREE speech, that in turn sells the paid event. He'll need to do several of these free speeches in a row at local events.

But it's a way to fill a room with genuinely interested prospects.

If your friend isn't willing to spend a couple weeks in a town (ahead of the FREE speech) to talk to different groups, then you are limited to Pay Per Click ads, meet-up notices, radio interviews (where he sells the free event).

If the audience is older (over 45) newspaper ads will work.

The other way, the way I did it, was call organizations that give events and simply ask if they want a free speaker. At that event, he gives his free talk to sell the main paid speech. Where he travels will be determined by where the event happens. He would give the paid event on the last day of the trade show, because most people have already paid for their flights.

Methinks you need a better hook...

We've had great success with past and present clients promoting our workshops to their clients... Can you adapt this idea...?

How about charities, non profits? They promote to their normal lists and you give them 50% of the proceeds, tax deductible. Or, if you have backend sales, 100%. People will attend to support the charity.

How about people who have frequent contact with new/budding business owners? Business Mentors, Business Coaches, Accountants... Could they promote to their lists for a profit share?