Taking a loan from the bank with interest is not permissible and the same ruling applies to guaranteeing the person who takes a loan from such a bank. Guaranteeing this person is cooperation in committing sin, while Allah (Glorified and Exalted be He) states:
...but do not help one another in sin and transgression. And fear Allâh. Verily, Allâh is Severe in punishment.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.