Who's responsible for a huge debt when no pre-nup was generated?

I met a retired couple who are facing a huge problem. Before they got married....one of them owed a huge debt but they never filed a pre-nup because they didn't know anything about them and they were in love.

The one who owes the money has a limited income making it impossible to make the huge debt payments and does not want the other one to be responsible for the debt. What's the best solution for this? Do they file married separate so the one, who may qualify for a refund, receives their refund or continue filing married joint? Is there another type of 'pre-nup' agreement they can file now so the other won't be responsible for the debt?

I'm just trying to help them out and I really don't know the right answer for them. Any 'professional' suggestions will be appreciated.

if you file for separation in it you would put your responsible for your bills and he is responsible for his own bills to and split the contents of the house or apartment in my day you might want to file married filing jointly this way you can split the refund or if its easier file married filing separately if thats better if the person who has the debt and little money it would be better if the person filed a separation agreement concerning the bills