Friend will be entitled to statutory sick but company sickness benefit is not a statutory legal entitlement but part of the employment contract. Company has a duty to satisfy itself that employer is entitled and take the opinion of its own professional advisor in doing this.
Issues involving stress, anxiety and back-pain are some of the most difficult for employers to deal with because opinions differ widely amongst medical experts.
At the end of the day, if an employee can't work, employer is eventually likely to go down a process to assess capability, and dismiss if there is no realistic prospect of return in reasonable length of time. Previous good record may delay the start of the process.