Likely cause of death?

[d_virginiana]

Garters can and will eat red wigglers; they're likely to spit them out, but if they're excited or hungry enough it's definitely not out of the question.
Gotta say, this doesn't look like any parasite I've ever seen or heard of though.

That snake still looks small enough to do the trick I've seen a lot of babies do, where they will actually inch up the silicone in the corners of a tank and end up on that little ledge near the lid. There are often wire edges there that could easily cause an injury, and it becomes really easy for the snake to get pinched or severely injured if the tank or lid is jostled at all...

[Sehayes1982]

Agreed on all fronts, your thoughts seem logical. I don't think he'll ever know, but I will encourage him to reduce probability of anything obvious becoming the cause in the future.

on a separate note, gartersnake.com and Ford in The Garter Snakes both reference the toxicity of red wigglers and how numerous babies died due to them. Unpredictable at best in terms of acceptance. But I agree that's not the culprit here!