[Valloric]

The file probably doesn't specify an encoding. Never import such a file; Sigil (or any other application for that matter) has no idea what encoding it's supposed to use so it falls back to the system default encoding. On your machine that matches the encoding of the file (purely by accident!), but on his machine it does not.

Make sure your HTML files specify an encoding, either through a <meta> tag, a BOM mark or in the <xml> declaration.

I plan on adding a subsystem that uses heuristics to guess the encoding in such situations, but that subsystem would best be described as an airbag; even when you have one, it's still a good idea to avoid smashing into walls at 80mph.