Once lock, Wi-Fi got disconnected automatically.

This is normal and expected behavior. If you are doing something, such as streaming Apple Music, Wifi will remain connected to keep the stream going with the screen locked. But if you are not using anything requiring a Wifi connection and your screen locks, Wifi automatically disconnects. You can avoid this however, by plugging your phone into charge and Wifi will remain, even if the screen is locked.

Sorry but ALL iPhones do this as lobsterghost1 explained better than me. So there is no solution, it is a feature to conserve battery power. And there is no Apple Team here, this is a user to user forum. If you want to contact Apple Support click on Get Support on the top right of this page.

Then your friend is either streaming something which keeps the Wifi connection going or his phone is plugged in to a power source, which as I said keeps Wifi from disconnecting when the phone screen locks.

Otherwise, what you have described is EXACTLY what your iPhone is supposed to do.

And Apple Team is not here on this user to user only forum, so they will not provide a response to you here at all.

You should be getting notifications IF you are connected to cellular data. Do you have Cellular Data turned on in Settings > Cellular? Then you scan below there to give permission to Apps you want to use cellular data in the absence of Wifi.

There is nothing to annoy you here. This has been the way iPhone has operated pretty much always. If you don't want Wifi to be interrupted when your screen locks, plug your phone into charge. Problem solved. Otherwise, turn the apps on you want to notify you using cellular data.

Again, it's not an issue that can be fixed, when your phone is doing what it was designed to do. As mentioned, you can plug your phone in to charge and it won't disconnect from Wifi. Or you can use Cellular Data to get notifications for the apps you want to notify you of something new.

That is absolutely normal. All iPhones disconnect when locked.