breach of rental contract due to duress and denial

My fiance had a roommate before we were dating. He had her sign a year contract stating she would pay $500 a month plus half the utility costs. She tore some ligaments at her work location and he was nice enough to accept partial payments from her (no rent with reservation letter though.) She bounced two checks to him and at the end of her lease he verbally told her he was raising the rent to $1000 with all utilities included in an effort to get her to move out (he did not know he could issue a 5-Day Pay or Quit or a 30 day notice to terminate the agreement to get her out at the end of the lease term.) She lived with him around a year and a half and owes him around 5-8000 in back rent. The defense she is claiming is duress and denial. How can she deny his claims that she didn't pay when he has utility bills and bank statements showing her partial payment checks or no checks from her? And wouldn't she have to show bank statements or proof of check or money order to say that she did pay him rent money? And as for duress, do rental agreements signed fall under the category of duress? She could have found other, cheaper places to live. He did pressure her to pay him his rent money but if she couldn't afford to pay, then she should have moved out. He did have a clause in his lease agreement that says the landlord is still entitled to take action later on even if he did not take action at the time there was a breach of the lease. Does anyone think he has a decent chance in collecting in this? I didn't think free rent was a given in America.

If she is sued by her former roommate she will have to show cancelled checks, receipts etc. showing that she paid rent, utilities etc. I don't think the duress argument will at all fly. I wouldn't even bring up the word in court.

How far into the lease before the tenant fell behind?
Does the lease agreement allow for any increase in rent before the full term?
Did the tenant agree or sign a new rental agreement with the new amount specified?
$5-8k seems like alot or arrearages for 1 1/2yrs ? Something doesn't add up with the landlords motives? What took him so long to evict? Any normal landlord wouldn't tollerate this extended time period.
Is the tenant a relative?
Were any payments made by cash? Did receipts accompany all payments?
What were the default agreements specified in the contract?
Deposits etc?
Is rent control in effect in your area?
Was the landlord legally able to rent out?
There are alot of Q's here to be addressed and maybe a hard lesson or two in being a landlord? Be sure and be familiar with landlord/tenant laws and rental agreements before you attempt this again. When in doubt, spend the money before hand to get good legal advise rather than chance a big loss later.
I would contact an attorney or your local legal aid for assistance. Good luck!