Her husband forced her to swear an oath telling the truth about a haraam action she did in the past. What should she do?

Praise be to Allaah. 

We
put this question to Shaykh ‘Abd-Allaah ibn Jibreen (may Allaah preserve
him), who answered as follows:

If
she is afraid of negative or harmful consequences, such as divorce or a
bad reputation, she can swear that she did not do it, and seek forgiveness
and do lots of good deeds.

She
does not have to offer any expiation (kafaarah) because this oath has to
do with something which is in the past, although it involves telling a
lie. The kafaarah is sincere repentance. And Allaah knows best.