Sheikh Khâlid al-Mushaykih, professor at al-Imâm University, al-Qasîm branch
It seems that, in this case, the application of henna to the hands of the deceased is being carried out as an act of devotion, under the assumption that it is Sunnah to do so.
If this practice is being carried out as an act of devotion or religious observance, then it will be an unlawful innovation that should be avoided. This practice was never established by the Prophet (peace be upon him), so it should not be observed as a religious rite.
All that is related to us from the Prophet (peace be upon him) is what we have from Umm `Atiyyah when she and the other women washed the body of her son. It says: “They washed him three, five, or more times if they saw fit to do so. In the final washing, they added camphor to the water and ordered his hair to be put into three plaits…”
It is not established that the Prophet (peace be upon him) ever ordered henna or other such substances to be used.
As for the deceased, there is no culpability upon her, since Allah says: “No one shall bear the burdens of another.” [Sûrah al-Najm: 38]
And Allah knows best.
Source: Islam Today