I did ww1 using those same figures once and I strongly urge him to take a look at Early War Miniatures trenches. They look great, and you can get them prepainted. I sold mine off or you ‘d donate them.
They are also scaled for 20mm / 1:72 and I found their metal figures compatible with the plastics so you can supplement for hard to find poses like grenade launchers.
Have him take a look: https://earlywarminiatures.com/product-category/ewm-scenery-products-designs/ewm-scenery-products-designs-trenches-infantry-and-artillery-positions/ewm-scenery-products-designs-trenches-infantry-and-artillery-positions-linked-trench-models-20mm-scale/