S is a vector space, so it must have a basis. Let T be a subset of S defined by linear combinations of basis elements with rational coefficients. T is a proper subset of S and is dense in S.
Not if his meaning of "basis" refers to the usual definition of the basis of a vector space. (That is, B is a basis of S iff every element of S is uniquely expressed as a finite linear combination of elements of B.) Of course, as stated, the problem is rather trivial.