As per my knowledge, you can do this by using web3, ethereum avascript API, which allows you to interact with an ethereum chain either via by RPC to a full node connected to the chain or by injected Web 3 (with metamask like plugin).The high level architecture would look like,
What you have to do is build your html interface, load a js file with web3 module imported and you can call
eth.getBalance(account_address) with the onclick enevt of the button and display the return value.
If you want it to be displayed when the user logs in, send the same call at the page loading time (document-ready) and get the return value.
You may refer this question, posts here and here as well.