Can you be forced to testify if you sign an affidavit for a friend's appeal?
Typically, you can be required to testify. If you sign an
affidavit, it means that you have evidence regarding the case, and
therefore could be served with a subpoena and made to testify.
However, even if you don't sign the affidavit, but it is believed
you have such evidence, you could be subpoenaed. Unless you have
some issue of privilege or have a 5th amendment problem, failing to
testify could put you in contempt of court.
However, new evidence is not heard on appeal, so neither
testimony nor affidavits are considered at that juncture, and this
is therefore moot.