At the first level of abstraction, SOAP is just fancy XML, and so you can just use the XML rendering stuff, maybe in conjunction with ERB, and parse it with Nokogiri and friends.
That said, when I hear someone talk about 'SOAP web services', they usually mean some part of the whole set of standards layered on top of it: WSDL, WS-Security, SAML, and so forth, in some combination. So you may need to clarify requirements with your customer here - how much SOAP do they really want, why do they want a SOAP API instead of a more modern, simpler, but perhaps less buzzwordy REST API, etc.
If they really want all the above, and you can't find good Ruby implementations to use, you may be faced with building a translation layer in Java or .NET. Expose a REST API from your Rails code, have your Java or .NET code consume it and expose it as SOAP. Not ideal, but probably better than trying to implement the SOAP standards stack in Ruby.
-David