"Even if your hair does not respond bad to the chemicals for years, one day it can start, because your hair is old and tired."
sorry Veemack, but that's just untrue. relaxed hair, colored hair, bleached hair, etc, they are damaged hair... after going through the chemical process. they don't stay damaged forever. like you said, you build up the protein, ph, and moisture back by doing the proper treatments. technically all hair is "just damaged", but also, no hair is damaged. hair is dead, you can't really damage it or make it healthy. "damaged" hair really means cosmetically unappealing microscopic hair and "healthy" hair is cosmetically appealing hair. and hair can get "damaged" in the easiest way, by wind, sun, water, even naturals have damaged hair. chemical processes and heat damage the hair on a higher level, but you can make it healthy again. the more damaged the hair, the more you have to take care of it, like you said. but it's not as big a deal as you make it. I see naturals get protein overload, moisture overload, heat damage, mid shaft split ends, detangling problems, dry hair problems, styling problems, etc. pretty much the same amount of problems relaxed heads have, more depending on the person. if our hair was perfect, shampoo, conditioners, oils, any products, they wouldn't have an affect on the hair. it's because our hair is damaged that products can fix our hair. if our hair didn't need any fixing, products wouldn't work. my sources: the science of black hair.
and I get the unpredictable part, but you apply relaxer to the new growth of your hair. the youngest part. how can the hair start responding badly because it's old and tired? do you mean the ends of your hair? but her hair is currently armpit/ shoulder length? how is it old?? and everyone hair's weakest point is their ends, not just relaxed heads. or do you mean old people's (sorry old people) hair? she's young! no need to worry about that for a while. ^_^'
I'm sorry for the criticism, not trying to sound harsh, but honestly, there's no need to make the relaxed hair is weaker speech every time you talk to someone with relaxed hair. have you ever had relaxed hair? I understand you might be taking it from your personal experience, but it gets repetitive. when replying to a relaxed head with a problem it always goes "your hair is dry/has splits/is breaking/etc because you have relaxed hair, and the relaxer process..." not every problem is because of having relaxed hair.
putting that aside, keratin protein is good for split-ends, but be careful! since you just got over protein overload, I don't recommend using it until your hair is at 100% again. I'd slowly introduce protein again by mixing it with moisturizing conditioners. but I'm not sure about the belief that only certain proteins cause protein overload, or that using a different protein won't bring you back to protein overload. that doesn't really make sense to me...
and yes, a comb with a seam weakens the hair, which can eventually lead to mid-shaft split ends.