Actually M10 is unambiguous. It means 10mm OD x 1.5mm pitch. "Standard" metric pitches are referred to in this way (M5 is understood to have a pitch of 0.8, etc.) You only have to specify pitch in a non-standard metric screw, 10x1.25 would be a fine-pitch 10mm bolt. (SAE Sizes always specify pitch 10-32, 10-24, 1/4-20 etc.)
Anyhow, you're right, I don't know for sure he needs an M10. He said he needed a 17mm, I assumed because he used a 17mm socket to get it off. A standard M10 Hex Head Cap Screw (HHCS) has a 17mm head width.
It's very possible, especially with a drain plug that the head and OD are mismatched. My coolant plug in the block has a 19mm head with about a 24mm fine thread. If this is the case, you won't find it in a hardware store unless they specifically sell assorted drain plugs in an automotive hardware section (like the one I've seen at Lowes).
Like I said, good luck with that. I'd probably spend the $2 at advance myself unless I happened to be making a trip to Lowes for another reason. But all I'm saying is that if the guy does find an assortment of drain plugs at the hardware store, it wouldn't be difficult to make sure you have the right size and pitch by comparing to the old plug (for pitch see if the threads mesh perfectly when the two are held side-to-side).
Also, if the pan is just buggered up because you cross-threaded it, Oreilly's loans tap sets.
