This obviously needs to be approached with care. I would recommend the implementation of a process to review the performance of the full board and each individual director. Each director can perform their own self assessment, and if you feel it is possible to pull off, an assessment of fellow directors (on an individual basis). Then, the lead outside director (if there is one) or Chairman of the Board of Chairman of the NCG (if there is one) can compile the comments and performance evaluations submitted by each director for each other director. Once the results are compiled, the same person can sit down with each director and review the self assessment along with the compilation of other director evaluations of the director being reviewed. In this way, hopefully objective data on the contributions of the director (or lack of contributions as the case may be) can be laid out, and the director may decide for himself or herself (with a bit of encouragement) to move on. If by the way the evaluations suggest something regarding the director in questions performance that is other than the premise of the above question, perhaps this director deserves a second chance.
Failing the above, I would confidentially approach the lead investor and let them know of your concerns without suggesting the director in question be terminated. I would engage in a dialogue such as how can we make this director effective, of what can we do to get some contributions from this director, and see where that type of dialogue goes.