I am wondering if a paladin can boost a summoned demon's saves against hostile effects at all – it's not exactly "friendly" in most cases – and, if the demon does get the paladin's +CHA to saves, if it can use that bonus to break free from control.
While this seems straightforward if you take "friendly" to mean the creature's true intent and beliefs, it is less simple when considering a creature that is being controlled. If someone has been charmed into working as an ally, for example, would they benefit from the paladin's aura?
Aura of Protection
Starting at 6th level, whenever you or a friendly creature within 10 feet of you must make a saving throw, the creature gains a bonus to the saving throw equal to your Charisma modifier (with a minimum bonus of +1). You must be conscious to grant this bonus. At 18th level, the range of this aura increases to 30 feet.
Summon Greater Demon
[...]At the end of each of the demon’s turns, it makes a Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name. On a failed save, the demon continues to obey you. On a successful save, your control of the demon ends for the rest of the duration, and the demon spends its turns pursuing and attacking the nearest non-demons to the best of its ability.[...]
Demons (MM 53)
A mortal who learns a demon's true name can use powerful summoning magic to call the demon from the Abyss and exercise some measure of control over it. However, most demons brought to the Material Plane in this manner do everything in their power to wreak havoc or sow discord and strife.