Technically, Visual C, as any plain C compiler on Windows, is usable. That is, it is perfectly possible to create applications with it that will use QuickOPC/OPC Data Client; the objects will be accessed via COM API, same as what we do in C++.
However, the resulting code will be large and ugly
. With use of some "helper" library, it can probably be made a bit nicer, but it will never be elegant, and easy to program.
Basically, even such such simple task that you mention (getting to instantiate the first object) will require several Windows API calls. Subsequent method calls will be a bit easier, but still bulky in terms of code needed.
I would not want the customer to make a bad choice. If he can achieve the goal with another library that is better suited, let him go for it. Only if our library has a feature that he absolutely needs and is not available elsewhere, then I would use it. In such case, I can help by providing initial guidance.
There are resources on Internet that would enable the customer (or you) to get a glimpse of what to expect, such as www.codeproject.com/Articles/13601/COM-in-plain-C
(although I think that the approach in the article can be simplified by using the MIDL compiler, the principles are in there).