A couple of thoughts:
Has your fiance considered alternative schools? This article suggests that $80k is the high end of the costs of a MSW:
http://www.nytimes.com/2011/07/24/education/edlife/edl-24roi-t.html?pagewanted=all&_r=0. Particularly given that you already expect to live apart, a cheaper but still great school could be a great option.
How long does your fiance expect to work until retirement? I'm only asking because, while a MSW may make a huge difference in a normal person's working career, it may not make as much of a difference for a mustachian who expects to work a much shorter number of years than the average person. This article suggests that getting an MSW adds $15,000/year:
http://socialworklicensemap.com/social-worker-salary/. If you consider the total cost of the loans (including total interest that will be paid) she will pay and the actual amount you'll get extra from that $15k after taxes, it will probably take at least 7-10 years for her to break even.
Do you have any equity in any assets? I used the equity in my car to pay off higher interest student loans more quickly. 1.99% felt so much better than 8.5%. Might be something to consider.
I also took advantage of every opportunity to reduce the amount of the minimum payment on each student loan I had by extending my repayment plan, signing up for the graduated repayment plan, etc. That helped me pay the bare minimum on the lower interest student loans, which enabled me to direct more money (that would normally go to the lower interest minimum payments on the 10 year plan) at the higher interest loans.
If I were you, I would probably use the $22k to repay the 6.55% interest private loan. That just makes sense financially. Also, private loans have less protection for borrowers than federal loans, so when the interest rate is the same, it's generally better to keep/take out the federal loan.
You may also want to look into private loans for her if you plan to pay them back very quickly, and into refinancing your student loans. I know Sofi offers some great refinancing rates, but they are variable rate. Also consider the protections (deferment, etc.) that you would give up by taking these private loans.
In fact, if you are EXTREMELY careful about money and expect to be able to pay a lot back quickly, you may want to even consider a 0% bank transfer credit card to finance whatever you know you can pay back within the 0% period.
One website that helped me consider different scenarios is
http://www.whatsthecost.com/default.aspx. It lets you input loans (or potential loans) and how much you can pay each month, and will tell you the total interest, time until payoff, etc.
Keep in mind that there may be origination fees with student loans that affect the math as well. I think 1%-3% of the total loan amount was immediately added to the balance of the loan when I took them out for grad school.
Finally, will you be married before she begins her degree? If not, make sure you would be ok if you pay her living expenses but never see the benefits of the extra income from the degree in the event that you break up. I know that is tough to think about and may be unlikely, but you should at least be aware of and ok with that risk.