Your friend will have to obtain a letter of invitation from an official organization, a relative, or from a friend. While there is no set format for these letters, there is some very important background information that must always be included. For example, the letter writer will have to clearly indicate the visitor's name, date of birth and passport number. The writer must also explain his/her relationship with the visitor, the purpose of their visit to the United Kingdom and the duration of their stay. It is also important to clarify whether the letter writer is a UK citizen and passport holder, a landed immigrant or someone on a long-term working or student visa.