I have to replace the front right ballast for my 2015 Lexus 450RX H. Should I also have them replace the one on the left at the same time, since the bumper will be coming off?

Hello, if you are (correctly) maintaining original equipment Lexus parts, I would only replace the failed ballast as there is no expectation that the other ballast should fail. Sure, you could always proactively replace it, but I do not think they are inexpensive? Have you verified it's a ballast problem, not a bulb issue?