Prefatory disclaimer: I left the WTB&TS in the early 1970s (before Armageddon), so my info may be a bit dated. For all I know Dubs today are worshipping the golden calves of Fred Franz and drinking swine blood on Nisan 14;
...but back in my day NO, a person would not be disfellowshipped for dating a non-witness. They WOULD have been considered... er, what's the term? SPIRITUALLY STUPID! That's right, they would have been "marked" as "spiritually stupid" and denied some of the many PRIVILEGES open to them. If they were a chick, the only privilege open to her would be scrubbing the toilet at the Kingdom Hall, and denial of that privilege would cause spiritual suffering which would lead to her repentance.
...but that was then...