Question

Can an NPC ally have a shiny Pokémon in a max raid?

Answer 1

They can only be shiny if they are pre-set in the game's coding

Also, Source 1 doesn't mention any Max raid NPCs with shinies

So no, they can't be shiny because they aren't coded to do so

Source 1, Source 2

Answer 2

No.

Here's a list of possible NPCs though.