The very short answer is it is probably illegal. The text of Penal Code 426.04 makes it illegal for a convicted felon to "possess" a firearm after conviction. Outside of your example, a convicted felon can possess a firearm at their home after five years of their release from prison, probation or parole, but that is not part of your question.
The issue will come down to whether the felon had possession of the weapon. POssession is defined as "the right under which one may exercise control over something to the exclusion of all others; OR the fact of having or holding poroperty in one's power."
If the felon is riding in a vehicle with another person who is licensced to carry the weapon, if stopped by the police for any traffic infraction, I believe you, the gun owner, are required to tell the police there is a gun in the vehicle. At that time, the officer will investigate whether the felon has "possession" in the eyes of the law. This is a situation that many times will end up, arrest first, determine answer to question later.
If the felon can avoid being around you when you have your gun, great. If not, he runs the risk of being arrested and having to prove he did not have possession of the firearm.
Tell him to be smart, and that the potential trouble can't be worth it.
I know that we talked about this but I will put it in writing since you have only one other answer.
Because the gun is in the waistband, clearly under the control of the one person - the license person who is not a felony - it is probably legal. The felon has no custody or control over the gun while it is in the waistband.
Should the gun be placed anywhere else in the car, who has possession becomes more blurry. Generally, the police will "put" the case on whomever is the closest - if that is clear. (For example, on the floorboard of the passenger's side - it goes to the passenger. In the back seat of the car behind the driver, it is the driver's. In the back seat behind the passenger, with no furtive gestures, if the car is the driver's car, then it will probably go to the driver.)
The issue has potential problems including whether the officer is truthful. The officer has a loose weapon in the car and the passenger is a felon. That is a bigger case to put it on the felon. (It would be a felony v. a misdemeanor if he put it on the driver.)
So, while legally I think one would be safe, if I were a felon, I would not ride with someone carrying a gun on their person (or anywhere in the vehicle.)