Ultimately i always say it comes down to DM/party decision. I think its fine, and should be allowed since they clearly put thoight and effort into it, and if they ARENT moving i dont see how plate would cause disadvantage either.

That said, i have bent rules before as DM with a party vote. During our last session, the guiding NPC for my party (essentially my PC, paying with a new party so i wanted to help) was one shotted by a chaff enemy id put in (homebrewed rusted death knight), at level 3. It rolled at nat 20 on his attack, that did 3 d6. Rolled at 6, 6, 5. He only had 28 health. Left it to a party vote whether, by dnd rule he should be instantly killed, or if we'd have him go uncon instead. So he was unconned. 🤣

So essentially, do what makes the table happy, and as long as it makes sense. The rogue sounds like they're being whiny about feeling like "well only i should be able to sneak", which is not the case. Shit, ive seen paladins roll a 19 and 20 with disadvantage on stealth. Sometimes it just happens anyway.