At the start of debugging, you have to assume everything is wrong/defective. One by one you break it down and prove what is working, and you'll arrive at the fault. Often it is expedient to begin looking for the easy-to-find problems like you did: broken solder joint, blown fuses, etc.
Likely candidates include burnt-out speaker coil, bad connector, or blown left side amplifier. Capacitors and semiconductors most likely.
Having every possible problem to choose from, no sane person could suggest a fix.
If you have the ability to measure voltages, take clear photographs or better yet take 'scope captures and have a schematic to post, somebody might be able to help narrow down the problem.
If not then I'm afraid your friend just tricked you into throwing out /recycling his old junk
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One thing I like about audio debugging (for stereo equipment) is that when you have one bad channel, you have one good channel. That means you can compare the differences between both channels (signal levels, voltages, etc.) and often narrow-down the problem.