" Never (rarely) start a solo on beat one, usually it is a done on beat 3 or 4
of 12th bar as a pickup.
End your solo on beat 2 of the 12 bar for a transition/or ending to get back
to vocals/ending riff. "
Basically he is hearing the last bar as a pickup to the next chorus. The chorus would be bar 12 to 11, and not 1 to 12.
Many folks do play like this. It moves the music forward.
It does not need to be a rule though. Many folks don't play like this. They might wait a few bars before they start. Take their time, start small, build it up over many choruses.
However, maybe he was complaining the guitarist was still ending his solo while he was trying to start the vocal pickup for the head out? That kind of overlapping is a undesirable.
Example: The soloist ends on beat 1 of the new chorus, expecting the singer to head out, but the singer was supposed to start a pickup phrase in measure 12 but couldn't because of a wanking soloist. M. 1 is too late to start, so now a whole new chorus has to pass by...
Cue the Bass Solo!!
Maybe this is your chance to have a bass solo on every song? When you're done soloing, play the walk-up / walk-down riff that cues the vocals-out in clearly and without doubt.
This is your chance. Seize it. ;-)
(I will add this.... to have a singer who can accurately count beats and bars of music is a rare and special thing. Sad, but true. So appreciate a singer who can count, and a guitarist who bungles solo endings that makes a bass solo on every song a necessity.)