Correct way to implement a Singleton field
IMHO, it's ok to use static methods if the object (HTTPHelper in your case) doesn't store data and can't have multiple states. 

A little suggestion: 

    private static final OkHttpClient client;
    static {
        client = new OkHttpClient();
        client.setConnectTimeout(TIMEOUT, TimeUnit.SECONDS);
    }


if you initailize your client in this way, you won't need getClient method at all!

Actually, what concerns me in your code is whether it must be used by multiple threads or not. What will happen of one thread call getHTML while client is used in another thread pending from answer?

Addition:
what does

private HttpHelper() { }


serve for? Do you want to forbid to create an instance of your class? If so, your class may be extended and instantiated, or your constructor may be accessed via reflection. Try this: 

private HttpHelper() { 
    throw new NonInstantiableException() 
}