Once hair is physically or chemically damaged, the change is permanent, and any treatment promising restoration is actually only a temporary cosmetic fix. Cutting off the heat-damaged ends before the damage travels farther up the hair shaft is the only true way to improve the condition of damaged hair.
Shampoos, conditioners, styling products and styling tools all potentially have the ability to make hair appear dry or limp. A clarifying shampoo and a couple of weeks of protein treatments are likely to improve the hair's condition if any of these temporary issues are at fault. If the condition does not improve, the damage is likely permanent.
However, in some cases attempting to restore good condition to the hair worsens existing damage. When this happens it is necessary to cut off more of the hair than previously required, which is sometimes more length than a person is comfortable with losing. It is safer to cut off damaged ends before more damage is done rather than wait it out and risk making it worse.
The best defense against heat-damaged hair is prevention. It is important to make sure the hair is clean and free of product residue, dirt and oil before using a straightener. It's also important not to straighten hair too often or to use high temperatures because these cause serious damage over time. There are a number of products available that reduce the damage straightening does to hair and many resources available online that teach people how to use heat-free straightening techniques.