Marriage by someone who isn't "scripturally free to marry" is very common in the JW world and is one of the few situations which leads to a guaranteed DF. Usual procedure is a reinstatement within 10 - 12 months following a couple of letters -sometimes a kindly elder on the JC will initiate reistatement. No repentance need be shown (how could it be?), just an element of sadness & depression (which is inevitable as most of the JW's friends have been shunning them over this time.)
Sometimes it is possible to overcome this - there have been a few creative methods used over the years to circumvent the rules. I recall one situation where a sister & her husband (both JWs) broke up & neither were free to marry. The sister then admitted to a JC that she had secretly comitted adultery while she was married some years previously but felt very sorry about it now & was privately reproved. This then freed her ex-husband to remarry. And when he had re-married , she was then free to re-marry.