As far as my knowledge goes, yes, a helm of opposite alignment can affect Demons.
As far as Demons&Devils go, they're "just" outsiders born on evil-aligned planes who're generally always evil and have the Evil subtype (see below) forced upon them. There is nothing mechanically preventing a demon or devil from having nonevil alignments (though they'd also count as evil on top of their actual alignment for spells that deal with alignment). They're still sentient and intelligent beings whose personalities can be changed (through experiences or force).
A subtype usually applied only to outsiders native to the evil-aligned
Outer Planes. Evil outsiders are also called fiends. Most creatures
that have this subtype also have evil alignments; however, if their
alignments change, they still retain the subtype. Any effect that
depends on alignment affects a creature with this subtype as if the
creature has an evil alignment, no matter what its alignment actually
is. The creature also suffers effects according to its actual
alignment. A creature with the evil subtype overcomes damage reduction
as if its natural weapons and any weapons it wields were evil-aligned
(see Damage Reduction, above).
Considering their circumstances of creation, their experiences during life and their society and their tendency to trade souls as currency or eat them, it's unlikely for them to change alignments unless major plot (or supernatural interference like the helm of opposite alignment) happens.