We can offer that plenary indulgence for a soul in purgatory instead. This would be a great act of charity and spiritual almsgiving to do during Lent. If we help the souls to be released from purgatory and enter Heaven, they will help us to get to Heaven as well.
You can obtain a plenary indulgence every Friday in Lent by saying the following prayer before a Crucifix after receiving Holy Communion. The other usual requirements for a plenary indulgence also apply:
1) Receive communion (ideally on the same day the indulgence act is done)
2) Go to confession (within a week before or after the indulgenced act)
3) That one pray for the Pope’s intentions (usually by saying an Our Father and a Hail Mary)
4) That one be free from all attachment to sin, even venial sin.
Prayer before a Crucifix
Behold, O kind and most sweet Jesus,
I cast myself upon my knees in Thy sight,
and with the most fervent desire of my soul I pray and beseech Thee
that Thou wouldst impress upon my heart lively sentiments of Faith, Hope, and Charity,
with true repentance for my sins, and a firm purpose of amendment,
whilst with deep affection and grief of soul I ponder within myself
and mentally contemplate
Thy five most precious Wounds;
having before my eyes the words which David in prophecy spoke concerning Thyself, O good Jesus:
“They have pierced my hands and feet;
they have numbered all my bones.”
On any Friday during Lent a plenary indulgence is granted the Christian faithful who, after Communion, devoutly recite the above prayer before an image of Jesus Christ crucified. On other days of the year the indulgence is a partial one.