All else equal 121 TPIC is typically better than 135 TPIC.
But since he already has 121 SIC and heavy international time, he'll probably do all right with 135 TPIC if he can get it faster than 121 TPIC.
He just needs to be prepared to answer the "why did you leave 121 for 135" question...
Right answer: I had an opportunity to get valuable TPIC experience.
Wrong Answer: Numerous wrong answers...
"I wanted to be home every night"
"I don't like time zone disruptions"
"I don't like the regimented 121 approach"
You get the idea, don't let them suspect you left 121 because you didn't like or couldn't hack the lifestyle.