Is a native ESL teacher better or much better than a non native?

I think how a teacher is judged can be done so along several different but rather broad lines. These could include

1. Qualifications
2. Experience
3. A natural aptitude for teaching
4. English ability / language awareness

The first two points can always be changed by an individual regardless of their nationality. You can do more training and you can seek out opportunities to find more experience. The third point is dependent upon the personality of the individual but is not necessarily dependent upon their nationality. Some people can't teach and those that can teach one particular level or age group might struggle if they were put into a different one. The final point is the one that perhaps separates out a bit more. The argument is often made that non-native speakers have a more explicit knowledge of the English language than us native speakers have but much like qualifications and experience this is something that we native speakers can always go out and gain via studying the language and teaching it. On the other hand a non-native speaker can't truly become a native speaker. You can get very close to being so but unfortunately your cards have been dealt in advance by the fact that you weren't born and/or didn't spend a significant portion of your life living in an English speaking country. Therefore, I guess if you were looking for the 'ideal' candidate to teach the English language it probably would be a native speaker who ticks all the boxes above. Of course there aren't many 'ideal' candidates out there so in reality you will find that some non-native speakers are better than native speakers at teaching English.

Can you believe it? a non native teacher has been staying and working at the same school, public college for 8 years?

Yes I can believe it. Non-native speakers have fewer opportunities than native speakers and so when they do find a place where they feel comfortable and accepted they will keep it. However, this example is slightly clouded by the fact that she teaches 2 languages. If the school were looking purely for an oral English teacher they might have treated her application differently.

In general it's interesting how you often find that non-native teachers stick it out at a particular school longer than native speakers do. Again this must be seen as a realization that there are not always that many good opportunities for them and if they find one they tend to grab on to it. You also tend to find that the long term teachers come from smaller more obscure countries. Serbia is not part of the EU and when your friend in jiangsu arrived in china then nor was Romania (joined 2007). Therefore on a global scale these applicants have far fewer options than native speakers as well as some other non-native speakers who come from larger countries.