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She can do a POA - but be careful with that.
Some CA's and some OC's may not want to deal with you, even though you have the POA. (experienced that)
Another thing that did happen to me (could happen to you) was the OC's started calling me every few weeks demanding payment on my brother's account! DB (he's really not that "dear" anymore) would verbally agree to payment plans and never follow through. It got to the point that I had to tell OC's remove the POA from the account and that I was sorry brother would not keep his word.
The other thing that happened was the OC's would call me asking for current contact information for DB.
All of the OC's were very polite when I asked them to stop calling me. And they all did stop calling me.
Some OC's/CA's may hold you liable for the debt since you made the agreement (even though it was on your fiancee's behalf) to pay off the debt. Not fair, but can happen.
My suggestion would be this - carefully research each company before contacting them as her POA. They all should require a copy of the POA in their hands before they talk to you - which means they will have her signature. If the companies have a bad rep, don't send in the POA. Some companies are great, will honor the POA, and respect you helping them get their money.
Be careful.
I was VERY careful when I called on DB behalf. I made it clear that I was there to help the two of them make a deal. Since DB wouldn't talk to them, I was willing to be the go between and take offers. The OC would give me their best offers. If it was something I knew he couldn't do, I would say so and work another offer. I was successful with several companies. The problem ended up being DB refused to follow through on payments. AND he REFUSED to sign any paperwork/deals.
I had the POA but didn't want to have my name associated with his bank account. I could have taken the money out of his account myself or authorized the OC's to do the same. But I didn't.
The CA's that had bad reputations - I would not send them a POA. And they would all talk to me without one. (Everyone talked to me - they wanted their money.)
For me, it had to be a give and a take. One of the "terms" I would ask for is that the OC not extend anymore credit to DB. I didn't want them to enable him to sink further into debt.
As you negotiate, make sure you take great notes. Names, ID numbers, telephone numbers, email addresses, and get the terms in writing FIRST.