I agree with Rayna.
An appeal process would take a long time and based on the information you've shared the refusal would most likely be upheld.

It's important to submit correctly all mandatory evidential documents.

Personally I feel that there may have been further doubts about the application. A missing piece of documentation such as Employer Letter would normally be made known to the applicant and a given amount of time allowed to submit one.
This has been the case previously.

Based solely on what you've shared here your friend needs to review all the requirements carefully and consider to make a new application.

Sorry to learn about the refusal. All here understand the emotional impacts