It's great that you are helping this person make this important transition. Good for you.
For best results, the date and time for the body to leave the house (or wherever the death occurred, i.e.
, hospital, hospice, nursing home ) should be determined by Tibetan astrology as should the date of the cremation. Generally, the body is left undisturbed for three days unless the astrology suggests otherwise. The body can be packed with dry ice if necessary during this time. If the body must be moved sooner than three days or sooner than the astrology recommends, the dead person should be explained the situation and asked pardon and permission before being moved.
I recommend you get in touch with Loppon Rechung at Mipham Shedra in Boulder, CO. He can possibly help get the astrology done when the time comes. This is one of the services he performs. I suggest you get in touch before the death happens so that Loppon-la knows who you are, what the situation is, and things are somewhat set up before the event occurs. Loppon Rechung advises people have their "bags packed and ready to go" before death occurs and not just wait till the last moment. He also teaches a great weekend class on death and dying from the point of view of Tibetan Buddhism, including how to handle and dispose of the body, how to read the Bardo Tho-drol (Tibetan Book of the Dead) to the dying and dead person, how to do the Amitabha sadhana, how to do phowa, etc. http://www.miphamshedra.org/
Good luck and best wishes.