Indian law (not British law) requires that the child born to Indian parents abroad and having multiple citizenship must choose which citizenship s/he wishes to retain, within six months of turning 18.
If s/he does not renounce any foreign nationality within that timeframe, their Indian citizenship automatically lapses.
See Section 4(1A) of the (Indian)
Citizenship Act 1955.
(1A) A minor who is a citizen of India by virtue of this section and is also a citizen of any other country shall cease to be a citizen of India if he does not renounce the citizenship or nationality of another country within six months of attaining full age.
So, the child will have two choices when s/he turns 18.
a) Renounce British citizenship (which also includes Right of Abode). The child would no longer be a British citizen and would not have the Right of Abode in the UK.
b) Renounce/allow Indian citizenship to terminate. The child would remain a British citizen and would need to apply for a British passport as their Indian passport (which would contain their Right of Abode certificate) would no longer be valid.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.