Basically, drill and tap a hole for a machine screw. Glue and screw the parts together. Done. You don't have to use a 6-32 machine screw. That's just what I had in the house.
Took me about 2 hours, but I made several mistakes. You can avoid those mistakes by looking through these instructions. I think it's about an hour of work if you have the tools and do it right the first time.
You'll need a bunch of tools. If you don't already have a drill press, drill bits suitable for metal, a hack saw, file, screw drivers, etc, this will be a very expensive project. If you're without tools or experience, you should seriously consider asking your friend to do it for you.
You could just buy the replacement part, but you'll still have to take the tripod apart. The replacement part doesn't come itself; it comes packaged with the upper leg part. Also, you'll have to figure out the vintage of your tripod. Bogen has been making this model for years and there are subtle differences. (If you do buy a new part, at least recycle the broken bit.) And these are just the practical difficulties.
The reason I chose not to buy the replacement that I'm philosophically opposed to thinking of everything as disposable. Disposable items make sense for certain applications---medical supplies, for example. But do we really need to have something disposable to wipe off the kitchen counter? Or eat off of?
Repairing the stuff you already have is green because it reduces the amount of stuff in landfills and promotes responsible thinking. (It's better than recycling because recycling is often promoted by plastic-producing companies---to dull the guilt of buying plastic stuff. Also, it's harder than you'd think to recycle plastic. And the environmental impact of the transportation involved in curb-side recycling programs may offset whatever small advantage there is to recycling glass.)