Strong experience in a specific language isn't necessary. I know it's anecdotal, but reddit started out being written in Lisp, then switched to Python pretty early on. Many great tech people I know have no problem moving from language to language, or framework to framework.
"when all you have is a hammer, everything looks like a nail"
you want your co-founder to choose the best language/tech stack for the job. A c++ developer is fine for a web-app if he's smart and is open to learning new languages.
