My left drive arm came free on my ride this morning. Seems like the bolt came loose and stripped some threads on its way out. I can’t tell if it stripped the bolt, the spindle, or both. I’m hoping to salvage this with just a new bolt and/or spindle if necessary. I have a praxis works chainring paired with Oval 520 crank. The LBS thinks that the Oval 520 left drive arms come with the bolt pre-installed and aren’t designed to be removed and replaced. What should I do?

If the bolt's stripped, it's easy to see. Inside the spinle is harder to see, but threading in a good bolt, you can feel if the threads are ok.
Is this a square taper crankset ?

Bottom bracket spindles are hardened steel, and don't easily strip. Check the threads on the mounting bolt or nut as they are more likely to strip than the spindle. They're also cheaper and easier to replace.

N.B. if you rode any distance with a loose crank arm, there's a good chance the arm may be damaged and will no longer stay secure on the spindle. Look for gouges or other damage where the arm mounts on the spindle.

Yes, I’m fairly certain the retaining bolt became loose and stripped out under pressure. Perhaps it was removed for lubing and wasn’t properly torqued when reinstalled. Yes, I believe it’s a square taper but it’s at the LBS right now.