One option is to use IMAP.
Enable IMAP in Gmail. And use a desktop client like MZLA Thunderbird (formerly Mozilla Thunderbird).
In Thunderbird, there is an option in Account (Gmail) > Composition & Addressing > ☑ Automatically quote the original message when replying.
Uncheck that and save.
I just tried that with my friend and it seems to work fine. My reply was added to his thread. I assume that is because of Message-ID.
Thunderbird can auto-configure a Gmail account based off of the E-mail address used during setup. Thunderbird will also trigger a two-step authentication process which will give it a third-party app registration in your Google account.
I'll also suggest that you limit the number of messages synchronized. As of today, Gmail gives the option to Limit IMAP folders to contain no more than this many messages.
I selected the lowest option--1,000--from the drop-down list.
I was interested in removing quotes because I wanted to avoid loading inline images in replies, and I wanted my replies to be more like contemporary messaging apps: no quoting. Imagine if text messages or WhatsApp quoted entire threads.
The only time I've ever been asked to quote was in public mailing lists where missing quotes confuse newcomers. And it is probably a good idea to use full quotes on business project threads.
Also note there is a throttle limit mentioned in the Gmail Help article:
Note: To avoid temporarily locking yourself out of your account, make sure you don't exceed 2500 MB per day for IMAP downloads and 500 MB per day for IMAP uploads. If you're setting up a single IMAP account on multiple computers, try taking a break between each setup.
Each device will have to synchronize. And staggering the setups will prevent you from being locked out because of the initial synchronizations.