I've tried performing C++ code injections on an apk (I didn't need a DLL as I could just place the file in the apk folder), however I don't know where to start or if I'm going on the right path. How would I target the process (the apk) and read/write its memory? I can't seem to find this anywhere - there are tons of tutorials on this for Windows, some for iOS through jailbreak, but nothing for android. The apk was mostly programmed by my friend (it's called Egg, Inc.) and he is well aware and okay with the fact that I am hacking his app for personal usage. It would be greatly appreciated if someone just gives the spark for the fire, or at least tells me if C# is fit for this situation or what my options are! Thanks in advance.
At runtime? You wouldn't. You're explicitly prevented from doing that. Its a giant security hole to allow apps to do this, so they're prevented on any modern OS. If your friend wrote this, ask him to make the changes or ask him for the source code to directly modify the app.
At compile time? You can unzip the apk (the apk is just a zip file) and decompile the bytecode. Then you can edit it, recompile, sign it with your own key, and install it.
(Side note: android is linux based. Dlls are Windows only. The linux equivalent is a .so file. However just putting one in a path somewhere would do nothing.)