How did your friend convince your friend group to migrate over to Matrix? I’ve thought about setting up one but my group isn’t so much into the “do it yourself techies” or privacy conscious crowd.
That said, one thing that did help a lot was having the server host a default client. Anyone can still connect to the server using any client they want, but now they can also go to the server's URL and get a good client without any hassle. We chose Cinny as the default client because it's solid and extremely familiar to any Discord users.