Well, it might not be true for other OSes, but in Windows a colon isn't allowed, either. It's one of Windows' prohibited characters for a file or folder, only used after a drive letter. So it actually shouldn't be allowed, given Ren'Py tries to work with multiple systems. But I get what you're saying; it's definitely too restrictive.
His problem is with update, which means he already have the game correctly installed, thus the path should be correct without colon or something. However, not all programs are designed with all the languages in the path, so although windows itself support it, the program does not.Doesn't even need to be extended; a standard ASCII colon in the path causes Ren'Py to shit itself.
Since he uses only English, it should probably be fine. Still a word of advice: use English alphabets plus _ ONLY for ALL path and nothing else. Space should also be fine in most cases.