A student posted my lab materials as a "project" on Instructables, and hid its origin as a lab assignment. How should I address this?
It seems to me that there are two orthogonal aspects to this:
Copyright
Attribution
Either of these would in my eyes justify talking to the student and/or to the websites he posted at.
Re 1, you will need to decide by yourself whether you want to stick with a narrow interpretation of your copyright. If you don't explicitly allow dissemination, then the student has no business disseminating your work, period. Slight changes to wording don't make this more ethical or more legal. Of course, you can just let it slide, but I'd argue it would be good if you explained this to the student as a "teaching moment" and required him to take the material down. If he does not comply within a reasonable time frame, I would recommend that you talk to the websites.
Re 2: I find it much more serious that the student does not even properly attribute the work to you. What could be his motivation for doing so? After all, he could just as well put your name prominently on the materials (which would still violate your copyright, which is why I consider these two issues orthogonal to each other). The only motivation that comes to my mind is that he explicitly wanted to pass the work off as his own. Will he put this on his CV as an "example" of his portfolio? He shouldn't do that, and he shouldn't get away with this. Again, I'd recommend that you talk to him and to the website, if he doesn't take the material offline.
In addition, I'd say that this second issue would certainly warrant at least discussing possible disciplinary consequences with him, depending on what possibilities your university offers. This is certainly comparable to straightforward plagiarism - he didn't do it to get a better grade, but quite probably to show off somebody else's work as his in a non-academic context. I'd say that the sanctions your student code spells out for plagiarism would be a good starting point for holding a discussion with this student.
In the current discussion there seems to be a somewhat of an agreement that the student maliciously misattributed your work to oversell his own contributions. Based on the information given that seems far from obvious to me (you know, "never attribute to malice what can be attributed to incompetence"). Yes, it is certainly weird that he uploads your code and design without saying that he only did (small) parts of the entire solution. However, it is entirely possible that the student really did not think about this at the time, especially given that your code is actually open source and redistributable. Your figures are not, and even for your code the student clearly needed to attribute you, but those points are actually quite subtle and it seems entirely possible to me that the students simply is not aware of that.
Question: How should I address this?
Should I say something to the student (who is not my student anymore)? If so, what? Should I take any further actions?
Yes, definitely, but unless there are strong indications to the contrary, I would keep all discussion under the assumption that the student just made a silly mistake and will be willing to fix it.
In the first instance, I would just send the student a mail and tell him that you stumbled over his upload and are dissatisfied with his attribution of your work. Tell him that you expect him to make obvious which parts are your copyright, and which parts are his own work (e.g., via headers in the source files, as customary). If you don't want your images to be part of the upload at all, tell him to remove them entirely.
Should he decide to ignore this request, what you can do next really depends on how important this is to you. Roughly in order of level of escalation, you can either let it go, keep pestering him per mail, talk sternly to him in person, send a mail to the platform and make them take it down, or contact the dean of studies (or whoever is in charge with student ethics in your institution). I would definitely not do the last, but all previous reactions are perfectly suitable.
My answer assumes that you did not attach any kind of license or copyright notice to your repository.
I have a feeling that this student has misunderstood your repo being public for being open source. It would be best to take a deep breath and explain the difference to this student. Not many people realize that a repository without a license defaults to standard copyright law. (i.e. You, the author, reserves all rights.)
So first, add a proper copyright notice to the repository to prevent this from happening again. Then go calmly explain to your student what it means when there is no license in a repository. I guarantee you this was an honest (and actually very common) mistake.
Step back and consider that it's very likely this student thought this was open source material and (I'm assuming) no license displayed detailing how the material could or couldn't be used. It's an open source world for these kids. Without a notice saying the author must be attributed, they likely didn't know any better. You're a teacher, so use the opportunity to teach.