Standard repair is to sleeve the mast and rivet both sections afterwards.
The sleeve is installed inside the mast, snugly fit, and if there's a mast track or any other irregularity inside that may need lots of work. Then both sections are heavily riveted (it can look like a checkerboard all around!) and off you go. Welding usually is not done because welding aluminum is harder and less reliable than riveting, unless you've got a good welder or a mast small enough to get into a welding shop, as your friend's might be.
Replacing the mast might sound easier--but the cost of shipping a replacement probably doubles it. There are a number of "more than chandleries" that can track down used boat parts, and if you can settle for "a spar about 30' long and four inches thick" or whatever the spec is, you may find you can do better by just buying a similar mast and boom, and then making accomodations on your rigging. (Which is due for a change every 10-20 years anyway.)
Personally? I wouldn't want the splice and extra weight all that way up the mast, I'd try to find replacement options.
Some boats actually are built new with spliced masts, or have spliced masts installed to replace broken ones, since conventional shipping and trucking stops at 40' OAL and if you need a 55' tall mast and don't have then delivered by the rail or truck load...you order it in two pieces with a factory splice.