We are working with a 3rd party developer on an app in which our sales people can use to create/update leads within our system. Unfortunately this vendor has never integrated with Marketo and is unfamiliar with it. I recommended we use REST API for this, but they are stating that it is too difficult and suggested, if possible, if we can use Mobile SDK. Any suggestions would be greatly appreciated,
You can only use the mobile SDK if you have a subscription to Marketo Mobile Engagement. It is also extremely unlikely the any of the mobile SDKs would be useful since they rely on either the IOS or Android runtimes which are not available on typical servers. Your vendor should use the REST API.
If this is an app run on IOS or Android then they should definitely use the Mobile SDK. As Kenny mentioned, you will need a special license for it.
You can take a look at the official & community supported libraries here: GitHub - Marketo/Community-Supported-Client-Libraries: Community Supported Client Libraries
I'd be wary of any library that hasn't been updated in the past month (that's almost all of the libraries at the link). There are expensive commercial libraries that also aren't caught up with the latest Marketo API releases and/or don't implement functionality correctly.
I'd recommend starting with a REST/OAuth library, sure, but not a "Marketo-aware" library. The REST API is not exactly difficult to use (that's the advantage of REST). But the devil is in the details of the interface, and broken abstractions create confusion and/or a reduction in available functionality.