Someone stole a gun and sold it. He was arrested in texas.How long in jail is he facing?

If the gun was stolen in another state then he has committed a federal crime.
Federal crimes are punished very severely and in his case punishable by up to 10 years imprisonment. He may receive minimum sentence of 15 years without parole if your friend has three or more prior convictions for a felony crime of violence (e.g. burglary, robbery, assault, possession of offensive weapons) and/or a drug trafficking felony.

If he stole and sold the gun in Texas then for his first conviction, punishment is by commitment to the Department of Corrections for five years.
For the second and subsequent convictions, the offense shall be considered trafficking in stolen firearms punishable by commitment to the Department of Corrections for not less than fifteen years.
For a conviction where the offender possesses two or more stolen firearms, the offense shall be considered trafficking in stolen firearms punishable by commitment to the Department of Corrections for not less than fifteen years.
If he commits any other offence while in possession of a stolen firearm there will be an automatic five years added to the sentence for every offence for which he is convicted.