If an user is saving their bitcoins on Electrum and an contentious fork happens, what determines what end of the chain will be followed? Does it all comes down to what the developers want to do with the software?
Older versions of Electrum (prior to version 2.9) follow the chain that their server follows. People can run Electrum servers on either fork. I think most people randomly choose servers, and those that do so are at the mercy of the server they connect to.
Shouldn't a system be developed that tells you a fork happened and lets you what chain to follow, or could this end in a total mess?
This is why I have never liked SPV wallets, you are the mercy of someone else when these things happen.
Electrum (version 2.9 and higher) is able to detect consensus failures between servers. It lets users select the branch they prefer. I don't think this has ever been used on the mainnet before, so hopefully it works correctly as tested when the time comes.
The bigger issue is that a contentious fork won't include replay protection. So it's not enough just to choose which branch to follow -- that can result in losing coins on the other chain. Many users will want to split their coins.