Firstly: 

There is nothing wrong with using foodstuffs for purposes other than eating and drinking, such as using them to take care of the body, because the basic principle is that using them is permissible unless there is evidence that it is forbidden. Allah, may He be exalted, says (interpretation of the meaning):

“He it is Who created for you all that is on earth”

[al-Baqarah 2:29]. 

Secondly: 

If this food has been treated so that it has turned into another substance, as is the case with cosmetics, there is nothing wrong with using it and that is not regarded as using food, because its former description no longer applies. This is what the fuqaha’ call istihaalah (transformation). 

In fact they stated that substances extracted from impure and other things are permissible to use, on condition that there are no traces of taste, colour or smell. 

Shaykh Dr. Wahbah al-Zuhayli (may Allah preserve him) said: 

Soap that is produced from the rendered fat of pigs or animals that were not slaughtered properly becomes permissible by means of this process of transformation and it is permissible to use it. 

It is not permissible to use lotions, creams and other cosmetics that contain pork fat unless the fat has been transformed and its essence changed completely. If that is not the case then it is najis (impure).

End quote from al-Fiqh al-Islami wa Adillatuhu, 7/211. 

For more information please see the answer to question no. 118266 

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: Is it permissible to use henna mixed with egg yolk to condition the hair? 

He replied: 

There is nothing wrong with that if it is beneficial. There is nothing wrong with using henna mixed with egg yolk or other permissible things, if that is good for the hair, such as helping to grow it long or making it smooth or other benefits, or helping to keep the hair and prevent it from falling out..

End quote from Fataawa Noor ‘ala al-Darb. 

http://www.binbaz.org.sa/mat/18554

He was also asked: What is your opinion about putting eggs, olive oil and honey on the hair, then washing it off in the bath, because it strengthens the hair? What is your opinion on doing wudoo’ with that on one’s head? 

He replied: 

I do not know of any reason why that should not be done if there is some benefit in it. There is nothing wrong with mixing eggs, milk, honey and the like and putting it on the hair, and it does not cause any harm if it is washed off in the bath, because there is benefit in that … like other things that are of benefit. But if it is washed in a clean place so as to be in the safe side, that is good, in sha Allah.  But as far as we can tell, if it is washed off in the bath, it does not do any harm, because in the case mentioned it is not regarded as food, because in that case it is no longer something that may be useful to someone.  

End quote from Fataawa Noor ‘ala al-Darb

http://www.binbaz.org.za/mat/18601

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Some women use some foods such as eggs, yoghurt and honey for their faces and hair, either as cosmetics or as remedies. What is the ruling on that? 

He replied: 

There is nothing wrong with that, because Allah, may He be exalted, says (interpretation of the meaning):

“He it is Who created for you all that is on earth”

[al-Baqarah 2:29].

So long as the matter does not go so far as disrespect towards the blessings of Allah (the food), in which case it is not allowed. 

From Liqa’ al-Baab al-Maftooh, no. 191 

And Allah knows best.