OP, I looked into how this could be done successfully for a bit and asked around with people who know more than me about tech tools.
I wonder if college technology departments are equipped to evaluate a computer for evidence of prior drafts, usage, internet traffic, app downloads, etc. They should be able to do this as well as to determine if any of these were overwritten or deleted. Since your student is at college and his college probably has a technology department, he could ask them to vet his computer and submit their findings as evidence that he did not cheat.
Otherwise, he should take the reduced grade and be grateful that’s the only penalty. Then, he should visit the tech department and ask them to help him remove the apps from his devices so he’s not tempted use them again.