An 'extragalactic' intruder may lurk among stars orbiting the Milky Way's black hole

How is weight of star S0-6? How suppose to live 10 billions years or more if it's weight is too big as a star from Main sequence?
S0-6 is a red giant, so it has left the Main sequence. From the paper:

S-stars are a collection of both early-type and late-type stars, and the early-type are B0–B3 main sequence stars with masses of 8–14 M_Sun.1),2) The late-type stars are G–M red giants with estimated initial masses of 0.5–2 M_Sun.
If S0-6 is indeed located at 0.2 pc from the SMBH, to our knowledge, this study is the first measurement of the chemical abundances of a late-type S-star.
So it is about the mass of the Sun, a star which may be about 10 billion years old when it becomes a red giant.

Seems to me the estimates are consistent enough.