Method is being called from the "wrong" class - undefined method error [duplicate]

Question

I have a CE Magento 1.9 application that is using a custom theme from Trego and a checkout extension from Webtex. The application was functioning but then after installing the checkout extension, I got the following error when loading the home page:

Fatal error: Call to undefined method Webtex_OnePageCheckout_Helper_Data::isOnepageCheckoutEnabled() in C:\Sites\mycompany\app\code\community\Trego\OnepageCheckout\Block\Links.php on line 6

Line 6 is as follows:

if ($this->helper('onepagecheckout')->isOnepageCheckoutEnabled()) 

Here's the relevant file, \OnepageCheckout\Block\Links.php:

<?php
class Trego_OnepageCheckout_Block_Links extends Mage_Checkout_Block_Links
{
    public function addCheckoutLink()
    {
        if ($this->helper('onepagecheckout')->isOnepageCheckoutEnabled()) 
        {
            $parent = $this->getParentBlock();
            if ($parent)
                $parent->addLink($this->helper('onepagecheckout')->__('Checkout'), 'onepagecheckout', $this->helper('onepagecheckout')->__('Checkout'), true, array('_secure'=> true), 60, null, 'class="top-link-checkout"');

            return $this;
        }
        else
            return parent::addCheckoutLink();
    }
}

My question: Why is the application referencing the Webtex_OnePageCheckout_Helper_Data class instead of the Trego_OnepageCheckout_Block_Links class for the isOnepageCheckoutEnabled() method?

Helper snippet from local/Webtex/OnePageCheckout/etc/config.xml:

<helpers>
    <onepagecheckout>
        <class>Webtex_OnePageCheckout_Helper</class>
    </onepagecheckout>
</helpers>

Helper snippet from community/Trego/OnepageCheckout/etc/config.xml:

<helpers>
    <checkout>
        <rewrite>
            <url>Trego_OnepageCheckout_Helper_Url</url>
        </rewrite>
    </checkout>
    <onepagecheckout>
        <class>Trego_OnepageCheckout_Helper</class>
    </onepagecheckout>
</helpers>