It is fine to remarry someone you were divorced from. I have a JW friend that divorced her husband twice.
The issue is dating a disfellowshipped person. If they are "scripturally" divorced, it would be technically considered the same as any JW dating a disfellowshipped person, which can be grounds for getting disfellowshipped.
On the otherhand, if they are not scripturally divorced it may be ok to get back together. I have never heard of such a situation, so do not really know.