The hair on the ends could be suffering from heat damage. The outer sheath of the hair cuticle forms overlapping scales. As long as this is not disrupted, the fiber is strong and appears smooth from root to tip. Over processing of the hair with chemicals or heat can sometimes damage the structure of these filaments and cause the hair to have a "frayed" appearance, be less manageable, or be more vulnerable to breakage.
Excessive heat from daily flat ironing can literally boil the follicles and damage the hard outer layers of keratin. The damage can often be reduced by the regular use of a good, well-formulated conditioner. But avoiding the damage in the first place would be a much better strategy.
She may need to rest the hair for a good amount of time from all heat styling. If the individual decides to continue to flat iron their hair, suggest that they always use an effective high-quality heat protectant before flat ironing (make sure that the ends are especially protected). Of course, it is important to note that extreme heat will always damage the hair in some way. So caution is needed with whichever method of straightening a person may individually decide to use. The best option may be to choose styles that accentuate a person's natural texture and curls and try to rest the hair from heat-styling everyday.
Another definition of "frizzies" or "frizzy hair" could simply be when the client has a curly/wavy texture to their hair and when their hair begins to revert to its natural state this can also be termed "frizzy". Many anti-humidity products, that prevent the reversion of curly hair after straightening, are called "anti-frizz" products.