Ok, this is a long post...
There is an ideal formula for every single head of hair. I mean, a comb, a brush, or a flat iron can break off your hair...if it is not utilized or prescribed correctly. There are different tools because everyone is different. No one head of hair is alike. With that being said, here I go.
There are different variables that are considered before applying sew ins,
1) The overall condition of the hair - this is pretty much damage control. If there is any fragility, or damage, poor porosity, anything like this, it must be addressed before a sew in is applied. If a protein treatment or fortifying treatment is needed, this has to be addressed first and corrective measures have to be taken.
2) The application being considered - there are several different
sew in methods. (Malaysian which is braidless, flat twists, cornrows, flat french braids, nets, tubes, etc) Just like there are different types of braids, kinky twists, etc, there are different applications of sew ins. There is more than one braid pattern or size to be considered for each head, you would not use the same application on every single head because everyone is different, you would not use a cross stitch sewing method on someone that doesn't need it. You use the formula that is best for each individual's head. Its like shopping, different trips birth different things. Every person is not going to purchase the same shirt or the same pair of pants. Everyone is different shapes, sizes, social levels. Everyone has a different formula.
3) The natural density and texture of the hair (Is your hair thin, medium, or thick)- This dicatates how thick your extensions should be. If your hair is rather thin, then you would either need less hair, or split the tracks before application, so that the weight is not too much on your natural hair. If you have fine 3C hair, then you wouldn't put 24 ozs of 24 inch yaky straight Hair in your head, if your natural texture is fine and density is thin,it would be more like 10 ozs with a hand tied weft. The extensions need to be becoming and supported by the method selected.
4) Also, extensions only need to be left in for 6-8 weeks. If you go past this point, it is never beneficial because you are risking your hair locking up, becoming overly stressed, etc.
I am a person that chooses to wear extensions, however I am not dependent on wearing extensions at all. When my hair is unveiled, I am always complimented on the body of my hair, the health of it, and questioned as to why I even wear extensions, however my thought on weave is my thought on any protective style. To me, they are no different. It is like a barrette to me. It is a way to dress up my hair, while protecting it. I am not my weave and I do not need my weave. I like my weave, and I love my hair. My avatar is my actual hair. It is cut into that style. I have worn extensions for years and so have a lot of my friends, and I haven't had ate up, broken off, or damaged hair from it.
The important part about installs is having someone that knows all of the variables and is able to calculate the right formula for you. I am third generation with hair so this is something that was birthed into me.
I have seen weaves done, I have done tons of them, and I have done all of my own, and I have yet to have one person that has lost their hair due to the process.
If there is a loss of hair after the process, then there are other things that would need to be figured out. Braids can be too tight, the weight could have been too great, the application could have been done carelessly.
If you relax your hair and it falls out, it isn't the relaxer that caused your hair to fall out, it is because the wrong strength was used, it was applied incorrectly, or it was applied with another chemical that wasn't considered at the time. If you comb your hair and pull out a chunk, it is not the comb that brought the hair out, but rather the teeth spacing, force used, etc. The same thing with this protective style, no install is the same. Naturals and relaxed alike.
Sometimes people say it doesn't matter who does your sew ins, if they are trained or not, but this can result in a bad situation. If the person doing your sew in does not wear extensions, they have no way of truly understanding them. If the person that just got their sew in done before you is leaving with thin edges and chinky eyes, run out as fast as you can! Your hair is next, and you will be left with the same miserable results. LOL. If your friend had understanding of extensions and she wants to put them in your head, just understand the risk that is involved with this. Then If you are still comfy with the ideam thenyou can let her, but I always say, look at their work first, inquire and research before you move on with this, then you won't end up in a situation where you can't enjoy such a wonderful invention!
These are pics of people that I know that have sew ins, but their hair is longer than the actual install. They all use them as protective styles and these are all people that are not bald or thinning from their installs. I can go on and on with these results. It is about what you know when it comes to extensions. There must always be care taken when your hair is being considered.
It took me sooooo freakin long to detangle that ish!! Never again for me 
