Background: In the next few months I will be applying for graduate school for a PhD program (or Master's if I have to go that way first). The degree will be Electrical Engineering focusing on Computational Electromagnetics (CEM). I have heard that the letters of recommendation are crucial, and should absolutely be written by people that personally know your work. Right now I have two candidates willing to write for me: (1) a tenure-track professor for whom I have worked for 2 years and although her expertise is not in CEM, the work I do for her requires a similar skill set. (2) a tenured faculty who has had me in a class, is a nationally recognized expert in CEM, and has worked on a project with me outside of school (specifically he was consulting where I was working over the summer).
Issue: For the third letter, the most readily available option would be my boss at my summer internship. He has known me for about 2 years now, he is a double major in Math and Comp Sci, and has a Masters in Comp Sci. He has also been doing work in CEM for a few years so he is an expert in that area as well. However, he does not have an appointment at a school right now (although he surely could), but instead works in industry. Keep in mind I will be applying to programs that are top 50-30 overall, but are top 25-10 in CEM (i.e. Ohio State, UTEP, U of Houston).
Would he be considered a strong letter writer, or should I seek another letter from a professor (who may not know me as well and/or may not be an expert in CEM)?
Note: although my internship is in industry, my job is to do research and development, so I am not questioning whether my background is strong, I am questioning whether the third writer's profile would be considered strong/adequate by the committees.