Set default radio button on webform
With Webform you can pre-populate form default values via the url with tokens:

Setting Default Values from the URL (7.x-3 and earlier)

The %get[key] supported by Webform allows form values to be populated from the URL.

Setting Default Values from the URL (7.x-4)

To set the default values of a component to use either:

Query Format: For urls formatted like http://example.com/node/120?key=bar use [current-page:query:key] to return key value.,

Args Format: For urls formatted like http://example.com/node/120/key use [current-page:url:args:2] to return key.

Taken verbatim from webform docs