That’s actually pretty common with many law school grads in the past decade and a half, and it’s not because they’re shifty or worthless.
Hell, contractors do that all the time.
Shifting between major firms isn’t uncommon, and outplacing from a major firm to a major client of a senior associate who isn’t going to make partner has been going on for years.
But 3 different corporate jobs totalling less than a year isn’t a good sign. And 10 or 11 years out should be time to make partner - or not.
For M&A, a few years at government pay at the FTC or DOJ would look good on the resume, but he might not have been able to afford the pay cut.