Probably not. Most Agreements to Judgment in Housing Court state that both parties are waiving their right to appeal. Read the Agreement carefully to see if this applies to your friend. He is certainly free to File a Motion to Stay the Execution, which could postpone the eviction date. There are numerous services available to low-income tenants, such as the tenancy preservation program. That would probably be a good place to start.
It is theoretically possible, but extremely difficult, to reverse a joint filing which was obtained through mediation. If there is to be a hope of doing so it would be very likely to require an attorney. You should seek counsel immediately to know if this is possible.
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If the agreement was entered into the court (which is seems as so), then your friend will probably be unsuccessful. He waived his right to appeal (if the agreement states so). Are you sure he had a lease and not a rental agreement?
No attorney-client relationship is created in responding to this question, and advice provided is based solely on very limited facts presented, and therefore may not be correct. You are advised that it is always best to contact a competent and experienced with the practice of law in the county in which you reside.