You guys are both right, you have to look and think about the revelent section before each flight. Yes, most 121 outfits specifically prohibit outside flying so most of this is moot but, in theory, if he/she did get approval, it would be possible but like this: Before a 121 flight he would have to make sure that flight would not put him over 100hrs for that month including ALL commercial flying. Before a 135 flight that number would be 120hrs. But if he had over 100hrs he could still do a 135 or 91 flight but could not do a 121 flight.

Example, if he flew 50hrs of 121, then 20hrs 91, then 20hrs 135 he could fly a 121 assignment if it wasn't over 10hrs. He could fly a 135 if it wasn't over 30 hours. Make sense?

Again, mostly just theory because no 121 outfit is going to let him fly 135 on the side. 91 maybe.

As far as compensated flying that seems to depend on who you talk to, for sure anything you accept money for, is compensated and counts for hours. But if you don't get paid for the flight but are reimbursed and don't pay anything for the flight, it's more of a gray area but most FSDOs will tell you that is compensation and will count. You didn't pay anything but you were compensated for the flight. If you disagree then call your local FSDO and get it in writing. Why take the risk if you are wrong and get ramp checked?

...semantics, but outside commercial flying. I don't know of a single place that prohibits all outside flying (and believe me, as an aircraft owner, I've looked).