<human>: Condense the text into a few key points

I [M/32] am considering paying off one of my girlfriend's [F/31] credit cards (VISA - $4,500) because she is being buried by the interest of 24% (APR).  She has a couple credit cards with smaller amounts of debt (~$2k more including student loans), but I am considering paying off only the VISA card so she can learn to budget and pay off the others while still paying me back without the larger interest hits.  I will have spreadsheet and plan for her to pay me back in a set time period.  I have over $15k in savings so I will still have some money for emergencies in the bank.
We rent together and split the bills equally.  I love this woman and absolutely plan on marrying her one day, but I've been taught to never trust the future.  
Should I pay off this one credit card?  Should I get her to sign something just in case our relationship ends down the road?
<bot>: Should I pay off $4,500 of my GF's $6,500 total debt to help her avoid big interest penalties?  Should I get her to sign something?