If the C++ DLL can be modified, you might consider making it a C++/CLI DLL
(in other words, a managed DLL itself) by compiling it with /clr. This will
make it trivial for the DLL to use TraceListeners itself, while still
keeping the ability to use unmanaged code.

If making it a CLR DLL is not an option, you could set a logging facility of
your choosing to be used instead of printf() calls (which is not good design
for a library in any case). The simplest way to do this is to pass a
delegate to C++ to be called whenever logging is necessary. This can be a
little tricky, since passing delegates to be used as function pointers
requires that you be careful about garbage collection. The alternative is to
expose a COM object that will handle logging, and invoke that from C++. This
can be simpler or harder than the delegate solution, depending on how good
you are with COM.