Time Machine and the dreaded negative sign
Yes, it's scary, and yes I'd be worried.  Why doesn't bob already own his own backups?  If it's because of a UID or UUID mis-match, what's preventing recreating this problem (perhaps in reverse) in the future?  If you can risk damaging the live machine I'd rather change Bob's UID and UUID on the live machine than on the backups, on the theory that there are probably other UUID mis-matches lurking on other disks.

In any case, scary as it is, if I were to try to fix I'd use the Finder if it were just a few folders that could be fixed with "Apply to enclosed items...".  If that was impractical, next thing I'd try would be to find files owned by whoever currently owns Bob's stuff and change only those files.  Going off of file path seems kind of risky to me.