Assuming that you actually mean computer engineer (as in digital logic and computer hardware) and not software developer, as so many people here are assuming, there are a lot of resources available:
For a gentle overview of how digital logic and CPUs work, Charles Petzold's book "Code: The Hidden Language of Computer Hardware and Software" is solid introduction. Nisan and Schocken's textbook "The Elements of Computing Systems" and the lessons at the NAND2Tetris site (https://www.nand2tetris.org/) are good if they want to get hands on with the subject.
There are a variety of robotics and electronics interfacing kits based off the Arduino and Raspberry Pi available through Adafruit and SparkFun. If they're more interested in the digital logic side, Digilent provides FPGA boards and test instruments intended for use in educational environments that also include tutorials https://store.digilentinc.com/the-zynq-book-tutorials-for-zy....
[EDIT] I've seen some recommendations for the video game Factorio in this thread and, odd as it may sound, it would not be a bad gauge of interest. Digital logic is all about getting the right signals to the right place at the right time and doing the right thing with them and Factorio definitely teaches analogues of that.
[EDIT 2] Another interesting project for them might be to build a computer from chips. Ben Eater has a design, tutorials, and sells kits for building a 6502 computer (Same CPU as the Apple II) on a breadboard: https://eater.net/6502. (Not sure I'd want to try that without a 5V tolerant logic analyzer but they're cheaply available nowadays, e.g. https://www.seeedstudio.com/Logic-Pirate-p-1750.html) Note that I don't vouch for any of these, they're just examples of what's available out there that your friend can investigate if it piques their interest.
For a gentle overview of how digital logic and CPUs work, Charles Petzold's book "Code: The Hidden Language of Computer Hardware and Software" is solid introduction. Nisan and Schocken's textbook "The Elements of Computing Systems" and the lessons at the NAND2Tetris site (https://www.nand2tetris.org/) are good if they want to get hands on with the subject.
There are a variety of robotics and electronics interfacing kits based off the Arduino and Raspberry Pi available through Adafruit and SparkFun. If they're more interested in the digital logic side, Digilent provides FPGA boards and test instruments intended for use in educational environments that also include tutorials https://store.digilentinc.com/the-zynq-book-tutorials-for-zy....
[EDIT] I've seen some recommendations for the video game Factorio in this thread and, odd as it may sound, it would not be a bad gauge of interest. Digital logic is all about getting the right signals to the right place at the right time and doing the right thing with them and Factorio definitely teaches analogues of that.
[EDIT 2] Another interesting project for them might be to build a computer from chips. Ben Eater has a design, tutorials, and sells kits for building a 6502 computer (Same CPU as the Apple II) on a breadboard: https://eater.net/6502. (Not sure I'd want to try that without a 5V tolerant logic analyzer but they're cheaply available nowadays, e.g. https://www.seeedstudio.com/Logic-Pirate-p-1750.html) Note that I don't vouch for any of these, they're just examples of what's available out there that your friend can investigate if it piques their interest.