Direct proof of use
The ZiS-2 appears in the 2014 Yemen Civil War record as a Houthi-aligned improvised vehicle-mounted gun. WWII After WWII reported in September 2018 that at least one ZiS-2 remained in service with Houthi forces, removed from its carriage and mounted on a Toyota pickup truck.
Tank Encyclopedia separately described a Houthi Toyota Land Cruiser Type 1f photographed on May 21, 2017 with a Soviet 57 mm ZiS-2 mounted on the back. That source places the gun within the Houthi practice of mounting captured or legacy heavy weapons on Toyota Land Cruisers during the Yemen conflict.
Sources: WWII Weapons in Yemen's Civil War, Type 1 Technical
Timeline
Oryx listed the 57 mm ZiS-2 anti-tank gun among towed artillery in its September 2015 handbook of pre-war Yemeni fighting vehicles, supporting the gun's presence in the Yemeni equipment pool available after the Houthi takeover and the start of the civil-war period.
By May 2017, a Houthi Land Cruiser carrying a ZiS-2 had been photographed, according to Tank Encyclopedia. WWII After WWII then reported in September 2018 that at least one ZiS-2 was still in Houthi service after conversion from a towed carriage to a Toyota pickup mount.
Sources: Handbook Of Pre-War Yemeni Fighting Vehicles, Type 1 Technical, WWII Weapons in Yemen's Civil War
Narrative
The cited Yemen evidence supports an improvised direct-fire role rather than a normal towed anti-tank battery. The ZiS-2 was a Soviet 57 mm anti-tank gun from the Second World War, but the documented Yemen example had been removed from its original carriage and placed on a light civilian vehicle chassis, trading stability and protection for mobility.
The likely provenance is legacy Yemeni stock rather than a modern transfer. Oryx documents ZiS-2 guns in pre-war Yemeni holdings, while WWII After WWII traces the Yemen ZiS-2 population to Egyptian use during the earlier North Yemeni civil war, after which some guns were destroyed, captured, abandoned, or left in the country. The public sources used here do not identify a specific battlefield firing, target, unit, or battle-damage result for the 2017-2018 Houthi-mounted example.
Sources: Handbook Of Pre-War Yemeni Fighting Vehicles, WWII Weapons in Yemen's Civil War, Type 1 Technical