You can't just go posting other's source code on the web without permission. There are other, better ways to deal with this asshattery.
There are two parts here, neither of which alone add up to the combined outrage (though both spurious):
1) Company A writes code to inject ads to documents, and Company B decides to inject these into pages from other people's services. Whether B got permission from A for this exact purpose we don't know, but it could just as likely be embedded in pages B serve themselves. Note, the injection part here is suspect, but unrelated to the DMCA notice.
2> Owner of said code (Company A) blows his lid that company property is openly accessible at GitHub. and uses appropriate tools to deal with it.
If this is one party injecting their own code into a HTTP session without consent, then objecting to the subsequent source disclosure then fine, let rip, but the context here is critical, and without a view of the source we can't assume more than we know: This is a hostile act, intentionally modifying content in-transit, and more than just compressing JPEGs for mobile network consumption, it alters functionality and potentially the security of the user's device.
He is well within his rights to ask for help from all and sundry in determining the potential harm, especially if (as it appears, I tried and failed) the file can only be downloaded from Company B's network and anyone willing to assist would be unable if not a subscriber. The Net Neutrality rules in India back him up, but unfortunately for him the DMCA covers GutHub under US law.
It does protect him from the asshatted letter that makes threats if he continues to exert his rights under Indian law. Flash Networks' conduct here is mixed, but Airtel is incredible, and so far they remain uninvolved and unthreatened.