Readstuff, with all due respect, you are almost right on this issue. As you noted, Wisconsin (and the other states you listed) are communal property states; however, the debt noted in this discussion would be pre-marital and that is not subject to communal rules. As such, this debt will remain hers and not become communal debt even after marriage.
However, Jen needs to realize that though her debt will remain hers, it can affect the overall marriage credit. Anytime a potential creditor runs the credit history, the negative on Jen's history will 'pop-up'. And, depending on the creditors rules, it could impact the lender decision (and could generate sub-prime, higher, interest rates), or even cause a rejection of the application.
Also, Jen needs to be aware that if the creditor takes the debt to court and gets a judgment, that judgement can be rendered against any non-exempt assets that the debtor has, individually or communally. This could include joint-owned assets like bank accounts. Then the courts would determine what percentage of the asset was owned by the individual debtor and 'split the baby' in order to pay the creditor.
To answer Jen's question ("will my fiance credit also be ruined?"):
No, 'your' credit history will never directly affect your fiance's, but it could affect it indirectly anytime that you make joint credit decisions.