All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Using a fake medical report is considered an act of cheating which Allah, The Exalted, has prohibited, since He said (What means):" O ye who believe! Fear God and be with those who are true (in word and deed)." [At-Twabah]. In addition, the Prophet (Peace be upon him) said (What means):"Shall I not inform you of one of the gravest of the cardinal sins?" We said: "Yes, O Messenger of Allah!" He (Peace be upon him) said, "To join others as partners with Allah in worship and to be undutiful to one's parents." The Messenger of Allah (Peace be upon him) sat up from his reclining position (in order to stress the importance of what he was going to say) and added, "I warn you making a false statement and giving a false testimony. I warn you against making a false statement and giving a false testimony." The Messenger of Allah (Peace be upon him) kept on repeating this (warning) till we wished he should stop." [Al-Bukhari].
Therefore, the aforementioned person should make sincere repentance , ask for forgiveness, give charity and never commit that sin again.
As regards his current job, he shouldn't quit if he is efficient.And Allah knows best.