Hi Jeff Matthews and thanks for the unique question!
If they don't have InfoPath on the client and you can't use browser-based forms ala SharePoint (since offline isn't supported), then you will need to use a different technology.
We have a tool that takes an InfoPath form template and renders it in the browser and it doesn't use SharePoint but it doesn't support offline save. For that you need a rich client application. InfoPath is the best rich client application on the market today for electronic forms, but you don't have it. So, you'll have to do a custom solution.
Not hard to do a custom solution that uses XML. You could even create a solution that is compatible with InfoPath so that down-the-road you have options if you decide to go the SharePoint or InfoPath route. Qdabra Software does custom .NET consulting and we have lots of experience with custom form solutions (that don't use InfoPath or SharePoint). Happy to provide an estimate for your project if you are interested.