My wife was asking me if there is any solution that can be applied to cloth before dyeing that will block the absorption of the dye.  She wants to write words on the fabric that will remain white.

Yes, there are many different resist products you can use for this purpose. 
The best but most trouble is hot wax, as in batik. See these two pages for examples:
Two fish snap-crotch baby shirt
Single-wax batik with low water immersion dyed background

There are many different water-based resists, which are easier to use than melted wax. They all work best if the dye is painted or sprayed on, instead of soaking the fabric in the dye. At all costs you must avoid all-purpose dye, such as Rit dye. It is a hot-water dye which requires extended immersion of the fabric in nearly boiling water. You must choose a suitable cold water fiber-reactive dye, such as Procion MX dye. Your local sewing store might carry Dylon Permanent fabric dye, which is another cool water fiber reactive dye and will work. For larger projects (anything more than a single garment), mail-order dye is a better deal.

The easiest to find water-soluble resist is Elmer's Blue School Gel, the type in the squeeze bottle, not the glue stick. It actually works quite well. Do not try to use white glue; only the blue gel will work. Squeeze it onto the fabric and let it dry thoroughly. Once it's bone dry, which may take a day or two, you can brush or squirt or spray on your cold-water fiber reactive dye. After the dye has reacted with the fabric, using soda ash, you can wash out the dried gel using very hot water. Look at the following page for an example of writing done using Elmer's Blue Glue Gel:
Immersion Dyeing with Water Soluble Resist


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