Citibank private student loan question

Through a series of unfortunate events, my husband has accumulated a significant amount of debt with Citibank. Out of desperation, we enrolled with a debt settlement company and have been working with them for the past year. We got an email today from the company saying they cannot settle with private student loans (after I had asked them repeatedly if we could). There have been a couple of people on this forum saying DCS offered to settle with them. Does anyone know for a fact if you can settle private student loans? We defaulted on the payments because the debt settlement company told us we needed to. We want to pay back our debt, but it is impossible to do so right now. We are living with my in-laws and have very few assets, but are willing to sell them to get back on track. Is this something Citibank might take into consideration? My husband called Citibank today and they said they would 'buy' back his loans if he wrote them a letter requesting they do so.