You cannot sign for them absent a written authorization from them -- Power of Attorney.
You cannot sign as paid preparer using TurboTax and you need a PTIN issued by the IRS and there may be additional requirement from the State ( both education, bond, license # etc. )
If they do not owe anything why don't you just wait and file when they return --- there is no harm.
If they are away from the country then their filing date without extension is June 15th.
