The woman's family is bringing the case before a civil court in Riyadh on Sunday after an Islamic court rejected the complaint, ruling that "a member of the religious police cannot be judged".
Al-Lahm said he hoped his client's case would help consolidate the role of justice in defending individual freedoms and human rights.
The case comes after al-Watan newspaper last month reported that attacks by the public against the 5,000-strong special force for the promotion of virtue and prevention of vice were on the rise.
The newspaper partly attributed differing views over the role of the Muttawa, to the changes undergone by Saudi society since the special force was founded several decades ago.
The interior ministry issued a decree in May 2006 aimed at reining in the religious police by requiring them not to interrogate detained suspects, as they had previously done, but to hand them over to the regular police instead.