Upvote:2
That is fine.
He has officially left the UK on the stated date and I am assuming he is Indian so has full right to be in India, there shall be no issue. The plane is not British territory.
The only possible potential iffiness is if this is a 3 month visa and he is leaving on the very last day. This will be noticed and make people suspicious if he tries to visit again soon after. But this is a minor consideration which normal people going for a once in many years visit shouldn't worry about it.
Upvote:7
Your uncle is leaving the UK before his visa expires; the fact that his departure date coincides with his visa expiry date should not be a problem unless for any reason his travel plans are disrupted and he cannot depart as intended.
You do not state in your question whether the length of his visit was consistent with the information he gave in his visa application. Staying in the UK for significantly longer than originally stated can cause problems for subsequent visits Cancelled UK visa on entry and UK visitor visa refused due to previously staying for too long and not having strong ties. How should I reapply?
UKVI may view being able to stay for 6 months as inconsistent with having strong ties to India - typically, most visitors would find it difficult to be away from jobs, family etc for such a long time. If the original application did not indicate anything like a 6 month stay, next time he applies your uncle will need to provide a strong explanation for why he decided to change his plans and how he was able to support himself while in the UK (to overcome suspicions of deliberately understating the visit length to increase the chance of approval, and of illegal working). He will also need to wait at least 6 months before applying again - the UK does not specify a wait period as such however the rule of thumb is to spend at least the same amount of time outside the UK as in it.