Regardless of the reptile, mixing wild caught and captive bred can (and usually does) result in your CB animals acquiring parasite infections and getting exposed to other potential illnesses. Think Europeans and Native Americans back in the day and the reaction to smallpox - a captive bred ball python has never been exposed to, and doesn't need to develop resistance to, any of the natural pathogens and other bugs native to North America. A wild garter snake can handle it no problem, but a ball python is unlikely to have the same resistances.

While there is unlikely to be any immediate effects (again, regardless of it being snakes, lizards, frogs, geckos, etc), eventually one or both of the animals often begin to decline, with the captive bred animal usually being the first to have issues. It is definitely possible to create mixed species terrariums, but it is also definitely a more advanced aspect of reptile keeping, and not ideal in a majority of situations.

In addition, snakes in particular are not well suited for mixed species terraria. They are predators, and when housed with species smaller than themselves, you run the risk of the largest snake eating whatever smaller creatures are with them. Unusual with ball pythons, true, but not unheard of. In addition, snakes do not normally seek out and remain in the company of other snakes for extended periods of time. It is always ideal to house snakes individually.

Aaaaand lastly, check that the 13 year old's parents know what they are trying to do. Nothing like rehoming the snake and getting a call in a week when their parents realize a new snake is in the cage, and the kid is asking for feeders that the garter snake never needed before. If you want the snake to have a good home, you may want to reconsider where it goes.

-Jen