The subtitles are in a format that isn't displayed correctly on your computer because it doesn't expect Greek text by default. Your friend's computer probably has the default codepage set to Greek.
- Possible solutions:
- You could change your system settings but that would affect all programs, so I wouldn't recommend that.
- In most media players or subtitle viewing plugins, you can choose the character set or encoding or codepage for subtitles. For Greek it should be "ISO-8859-7" or "Greek (Windows)".
- Convert the subtitles to Unicode. That is a universal encoding for (almost) all languages. Some editors/word processors have that function.
For example, you can open the subtitles file in Winword. When it asks for the encoding, choose "Greek (Autoselect)" or "Greek (Windows)". Then save it as plain text and in the "Save as" dialog choose the encoding "Unicode". (What Microsoft programs call "Unicode" is a variant of Unicode called UTF-16. If that doesn't work with your player, try "UTF-8" instead of "Unicode".)
edit: Don't use "Unicode" (UTF-16), use UTF-8. It's the quasi-standard, files are smaller, and opensubtitles supports it.