With the application I had attached an offer letter from a company
stating that I have a job offer open starting 3 months later.
This however has been tagged as non genuine, although the company is
registered and is run by someone I know.
There is a difference between the company being genuine, and the letter being genuine. People submit falsified bank statements all the time - in that case as well, the company (bank) is genuine but the bank statement is falsified.
The rejection notice unfortunately does not give you details as to why specifically they considered it as non-genuine.
The main problem here is that you have been rejected for deception - which is a very serious thing for visa applicants. Therefore, it is an uphill battle for you to turn over this refusal.
I would also make sure that any future applications leave no room for ambiguity as far as your documents are concerned, otherwise you may risk a 10-year ban.