Dear Nikita
As for your friend's brother, gifting to your friend would only be an application of income and he would have already suffered tax in his resident country. As for your friend, as pointed out, it would be a gift and being a "relative" as defined for this purpose, it would be exempt from taxation.
Documents relating to "purchase of a motor vehicle which requires registartion by a registering authority" is one of the transaction which mandatorily requires mentioning of your PAN.
Further as per Rule 114E, Cash deposits aggregating to ten lakh rupees or more in a year in any savings account of a person maintained in that bank is one of the group of transaction required by the banks to be reported in their AIR reporitng. So here, as per my knowlegde this transaction won't fall under this.
Also, though you mentioned so in the heading, I hope you do not intend to pay in "cash" being a material amount. Let your friend's brother transfer from his bank account to your friend's bank account and thereafter, he can make the payemnt to the selling car dealer.
Note that the remittance from the NR brother to your friend may be compliant with FEMA.
Regards
Ajay