[Phanx]

Basic code to spam the UIErrorsFrame with "Follower Name @ 42% HP!" while your follow's health is under 50%.

local alertFor = {
    ["Follower Name A"] = true,
    ["Follower Name B"] = true,
}
local f = CreateFrame("Frame")
f:RegisterUnitEvent("UNIT_HEALTH", "focus")
f:SetScript("OnEvent", function(self, event, unit)
     local name = UnitName(unit)
     if not alertFor[name] then return end
     local hp, hpmax = UnitHealth(unit), UnitHealthMax(unit)
     local percent = hp / hpmax
     if percent < 0.5 then
          UIErrorsFrame:AddMessage(format("%s @ %d%% HP!", name, percent * 100)
     end
end)

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