Is it legally right to transfer my friend mother's property to elder son's son (grandson)?

Your friend's mother is the absolute owner of the property as per your query.  So she can gift the property to whom she wishes to give through a registered gift deed.

Dear Sir, 

Thank you very much for your kind reply. God bless you and your family. 

Since your friend's mother is  absolute owner of property so she has every right to transfer the property as per her wish .No body/person can compel her to transfer the property as per his wish.She can gift ,sell or bequeath the property to any one.

Your friend's wife's mother  inherited the property from her father after which it becomes her own and absolute property,,i.e., her self acquired property.

In this sitution she can transfer her property to anyone of her choice or desire.

If she wants to transfer this property in favor of her grandson as a gift  deed, she can very well do so out of love and affection and the minor child's mother can be appointed as legal guardian to the minor son for this property till the child attains the age of majority. 

Her younger son cannot challenge this trransfer legally.

I fully agree with experts, she is absolute owner and entitled to transfer to anyone of her choice.

She can execute a Gift deed to whomsoever she desieres .