Do chem. relaxers & straightening irons damage hair?

My wife is 1/4 African, 1/4 Sioux, 1/4 Dutch and 1/4 French. Her hair is fine and naturally curly, and is bra-strap length, although she'd like to grow it to her elbows. About every 3-4 months for the past 15 years she has gotten it chemically relaxed, which turns it from tight ringlets that tend to frizz, into waves with a lot of volume. What she'd really like is smooth, straight hair. Every other time she has it relaxed, she has a clear cellophane treatment applied, which adds nice shine to the rich medium brown color.

Because of how long it takes to air-dry, she usually washes her hair only once a week (which really isn't often enough). A few hours after washing, she winds it up into a "snail" on each side of her head, and sleeps with it that way. On subsequent nights, she braids it up, which unfortunately introduces more wave than she'd like. Lately, I've been setting it on large rollers and putting her under a dryer bonnet (on medium heat) immediately after she washes it, which helps straighten it and allows her to wear it down the next day. At night, she now wraps it around her head in a sort of "turban", held in place by a net scarf. In the morning it comes out much smoother than it used to with the braid.

The last time she had the relaxer done, the stylist used a straightening iron immediately after the relaxer, and her hair came out looking literally the best it has ever looked - smooth, straight and shiny. My question (finally!) is this: how much will the careful and professional use of a straightening iron damage her hair? I'd hate to think that she's trading short-term success for hair that will start breaking off from heat damage. On the other hand, she is now very happy with hair that she has always hated in the past; and I'd gladly pay for her to be able to visit her stylist weekly for a washing and straightening - an opportunity she is happy to take, so long as it won't have disasterous effects.

Can anyone speak from professional or personal experience as to how much of a risk of damage there is from straightening irons, or sugest ways to lessen, prevent or repair that damage? Thanks in advance for any responses. Please feel free to e-mail me off-list at RHaines427@aol.com.