Rulings
Q1: What does a "your opponent can't reduce digivolution costs" effect do, exactly?
This effect prevents digivolution costs from being reduced when the costs are to be paid. This effect affects the opponent player.
Q2: Can my opponent digivolve a card using a "digivolve without paying the cost" effect while a "your opponent can't reduce digivolution costs" effect is activated?
Yes, it can be digivolved. "Digivolve without paying the cost" effects aren't effects that reduce the digivolution cost.
Data Sources
EN
Official English Card Listworld.digimoncard.com
Lv.3 Tentomon ST20-07 C Rookie | Vaccine | Insectoid/ADVENTURE [Digivolve] Lv.2 w/[ADVENTURE] trait: Cost 0 [Opponent's Turn] Your opponent can't reduce digivolution costs.
A
Official English (Asia) Card Listen.digimoncard.com
Lv.3 Tentomon ST20-07 C Rookie | Vaccine | Insectoid/ADVENTURE [Digivolve] Lv.2 w/[ADVENTURE] trait: Cost 0 [Opponent's Turn] Your opponent can't reduce digivolution costs.
W
DCG Wikidigimoncardgame.fandom.com
Lv.3 Tentomon ST20-07 C Rookie | Vaccine | Insectoid/ADVENTURE [Digivolve] Lv.2 w/[ADVENTURE] trait: Cost 0 [Opponent's Turn] Your opponent can't reduce digivolution costs.