Why is helium consider as a stable element?
Under normal conditions Helium will not react with anything because it has a complete outer electron shell.
No it does not. Being in group 18 (the halogen group), helium is a very stable and inert element. Its electronic configuration is 1s2 which means it has the stable duplet structure, one of the noble gas configurations. Thus it has neither a tendency to gain or lose electrons, nor is there any necessity to share them with others, which is why helium exists as a monatomic element in the atmosphere.
Helium is considered a stable element (you can tell because it is a Nobel gas, in group VIII of the Periodic Table), as it has a full compliment of electrons in its energy level. Ionic bonds will only be formed when a metal and a non-metal lose/gain electrons and become stable as a compound. There is no point in helium losing or gaining electrons with other elements because it's already stable.
8 valence electrons are needed for an element to become stable, which is why the noble gases do not interact with any other elements. They are already stable. However, the exception is Helium, the first noble gas, which only has two valence electrons. 8 valence electrons are needed on the 2nd and 3rd valence shells for any molecule to become stable.