I just spoke to Sibelius UK tech support. According to them the Kontakt Player 2 bundled with Sibelius 5 will only read libraries which are specifically formatted for Kontakt Player 2, so it won't read the VSL Horizon Series Concert Guitar Kontakt library.
However, Kontakt 2 (the full version of the sampler) will happily run inside Sibelius 5 so if you buy it you can access the library that way. It's arguably a good investment because nowadays many companies release their sounds in Kontakt 2 format. Hopefully VSL will one day release Concert Guitar (and its hairy companion, the excellent 'Overdrive' rock guitar) as Vienna Instruments?
However, Kontakt 2 (the full version of the sampler) will happily run inside Sibelius 5 so if you buy it you can access the library that way. It's arguably a good investment because nowadays many companies release their sounds in Kontakt 2 format. Hopefully VSL will one day release Concert Guitar (and its hairy companion, the excellent 'Overdrive' rock guitar) as Vienna Instruments?