Christina,
Before you make a decision, I recommend that you go to the Mexicana sub-forum on the kingsnake forum. The variations in what they call the Thayeri kingsnakes are incredible. Some of them look similar to milksnakes. Others have a unique look that I think is as pretty as that of any other snake. These snakes have a reputation as being very gentle, as being great eaters, of being somewhat visible, and for stopping at about 3 feet in length. I'm pretty happy owning just two snakes, but I think my first choice for a third snake would be one of these.
I think part of the decision depends on what you want in an animal. Do you want a snake that you're going to handle much? If so, what do you like to feel? If you like holding a big-bodied snake, a ball python would be a good choice. If you like a medium-bodied snake, then you're back in the corn and kingsnake range. I think the Thayeri kingsnakes are somewhat small-bodied, but I don't know all the breeds well enough to recommend a good small-bodied snake. Do you want one that will be visible? I love my California Kingsnake. He's a very nice animal, but he spends much of his life hiding under a water bowl. That's just the way that God made kingsnakes (or kingsnakes evolved or whatever explanation you find least offensive). Likewise, my ball python spends much of her time hiding. In some ways, it's very gratifying to see her using the hides that I provide. I see a little bit of her body underneath something that I bought for her, and I feel that I've done a good job providing for her needs. However, I don't get to see her out and being active very often. If your friend is experienced with snakes, she might be able to run you through better questions that would help narrow your search.
Good luck,
Bill
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It's not how many snakes you have. It's how happy and healthy you can keep them.