Can inmates file taxes to claim stimulus check

Your friend may qualify for the Free Edition.  Tax experts are available here.

Individuals who were incarcerated during some period of time in 2020, may, indeed, be eligible to receive the first and second stimulus payment.  This website states:

Because Congress did not exclude people in prison or jail, the IRS had no choice but to issue the payments to incarcerated people who otherwise qualified. Others who made this same argument ultimately prevailed in court and incarcerated people began to receive stimulus checks.

The taxpayer must meet the IRS eligibility rules:

You must have a valid Social Security number, and you can’t have been claimed as a dependent on someone else’s 2019 tax return.  In general, taxpayers without an eligible Social Security number are not eligible for the payment. 

See also the IRS Get My Payment tool.