If a person dies from being burnt and it is possible to wash him, then he should be washed. But if there is the fear that the body may be damaged or caused to disintegrate by washing, then water should be poured over him; if there is the fear that this may cause the body to disintegrate, then tayammum should be done if possible. 

Ibn Qudaamah (may Allaah have mercy on him) said: The one who has been crushed bya falling wall, the one who has been burned and the one who has drowned should be washed if possible; if there is the fear that washing may cause the body to distingreate, then water should be poured over them without touching them. If there is the fear that water may cause the body to distintegarte, then they should not be washed, but tayammum should be done for them if possible, as in the case of a living person who may be harmed by water. If it is not possible to wash the deceased because there is no water available, then tayammum should be done, and if it is possible to wash some parts and not others, then those parts that can be washed should be washed, and tayammum should be done for the rest, as is the case of the living. 

End quote, from al-Mughni. 2/209. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: 

How should ghusl be done for a person who died in an accident and his body is disfigured and some parts may have been cut off? 

He replied: 

It is obligatory to wash him, just like anyone else, if that is possible. If it is not possible then tayammum should be done for him, because tayammum takes the place of washing with water when that is not possible. 

Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah, 13/123 

In Fataawa al-Lajnah al-Daa’imah (8/371) it says concerning ghusl for a person who died in an accident and whose body is cut in pieces: 

If it is not possible to wash him, then tayammum must be done for him, because of the general meaning of the verse (interpretation of the meaning): 

“So keep your duty to Allaah and fear Him as much as you can”

[al-Taghaabun 64:16]

because Allaah has prescribed tayammum for purification from both major and minor impurity when there is no water available or when one is unable to use it, or when using it will cause harm. 

And Allaah knows best.