A Treasury bond is a promise by the government to pay you a certain amount of income every year, and to pay back the principal at the end. For example, a 20-year bond worth $10,000 might pay $450 every year for the next 20 years, and return the $10,000 after 20 years.
Now, if you hold such a bond, and inflation rises, the $450 payments will be less valuable. And if interest rates rise, a bond paying $450 a year will be worth less than $10,000, so that you cannot sell the bond for its original price. (The bond fund, which holds lots of bonds like this, does sell them before maturity, as it prefers to hold only long-term bonds and thus sells them when they become short-term to buy new long-term bonds.) These are the risks of holding long-term bonds, which is why they usually have higher yields than short-term bonds.
You might look at Inflation-Protected Securities Fund, which has a yield of 1% above the CPI, instead of a fixed-dollar yield. It thus reduces the inflation risk; if prices double, the distributions will also double. (It doesn't eliminate the risk, since your mother is not an average consumer; the prices of the things she buys may rise more or less than the CPI.) It is still vulnerable to fluctuations in value; it lost 3.04% in its worst quarter and failed to keep up with inflation in 2005-2006 because of rising rates. Thus it is not perfectly safe, but it is a reasonable place to park money which is being spent over a fairly long time.
