Is it permissible for a drug addict to wash the deceased, and is the questioner sinful for allowing him to participate in washing her father?

There is no sin upon you for permitting your son-in-law to assist the one performing the ghusl, as justice (adalah) is not a condition for the one performing ghusl nor for his helper according to the majority of scholars, even though you were more entitled to the task by religious law. It is permissible for you to waive your right to wash the deceased. The person most entitled to wash the deceased is his وصي (executor/guardian), then the closest male relatives (عصبات) in order of proximity.

Scholars have differed regarding the condition of justice (adalah) for the وصي, with the Hanbalis stipulating it.

The washer should be trustworthy. It is recommended that no one attends the ghusl except the washer and those whose help is indispensable. The wali (guardian/closest male relative) may enter even if he does not wash or assist.

As for smoking, it is forbidden due to its harm. You should strive to guide your son-in-law through good dialogue so that he abandons what harms him.