If it's a DF group and you can roll need on something, you can roll need on it. That's the only rule there is, written or unwritten, and anything else people claim is a rule after the fact is just stupid bullshit to justify a tantrum over being out-rolled.

Totally different issue if it's a premade group via Party Finder with loot rules agreed in advance, but you don't invent rules for DF groups.