Mr.nimasang, you have to apply for the Hong Kong visa through the Immigration Department of Hong Kong SAR Government. The Chinese Embassy in Kathmandu can only issue Hong Kong visa not more than 15 days, you can send the visa application directly or through the local sponsor (your friend) in the HKSAR to:
Receipt and Despatch Unit
Hong Kong Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, Hong Kong

The requirements are:
Application Form for Entry for Visit/Transit in Hong Kong (ID 1003A)

The applicant's recent photograph (affixed on page 1 of the application form ID 1003A)

Photocopy of the applicant's passport

An invitation letter in which your friend agrees to provide financial support and accommodation, and one copy of the your friend's HK ID.

Your friend as the sponsor also need to provide:
Application for Entry for Visit/Transit (Sponsor) in Hong Kong (ID 1003B)

Photocopy of the sponsor's Hong Kong identity card

Photocopy of the sponsor's travel document containing personal particulars, date of issue, date of expiry, and the latest arrival stamp/extension of stay label in the HKSAR (for non-permanent resident of the HKSAR only)