Your wife's separate debt is her responsibility, but a divorce court can consider her debt and how the debt was incurred when devising an equitable distribution of the marital estate.
If you wish to try to enter into some sort of post-nuptial agreement in relation to marital (and separate) assets and debts, I would urge you to do so with the help of a lawyer - and for your wife to have her own, independent attorney advising her.

