[Fizzlemizz]

You should be able to do something like changing:

local function Mixin(baseFrame)
    baseFrame.UnitFrame.BuffFrame.ShouldShowBuff = newShouldShowBuff
end

too:

local function Mixin(baseFrame)
    if not UnitIsEnemy(baseFrame.UnitFrame.unit, "player") then return end
    baseFrame.UnitFrame.BuffFrame.ShouldShowBuff = newShouldShowBuff
end

So friendly units don't receive the filter. There may well be a better way.

For documentation, WowPedia is probably the best other than asking questions here or one of the other WoW UI focused websites like the WoW UI forum or Discord