What will most likely happen is that you will develop bad technique. When you practise under a coach they can see and correct your mistakes quickly, but if you are practising at home you can annd most probably will practise your mistakes over and over and over so they are very ingrained and hard to break. This will make getting your back handspring take much, much longer. I have seen people take years to learn a back handspring because they practised incorrectly at home. Even if you do get you BHS yopu will find it very hard to progress on from there if you have bad habits andyou wont learn how to do a back tuck out of it for a long time. I know it is really hard to wait and you want to learn the skill now, but in the long run you will learn it much faster if you wait for your coach to teach you.
The worst that can happen is that you can land on your head or neck which could permanantlt paralyse you (and you wont ever be able to do gymnastics again) or it could even kill you. This can even happen if you are on a trampoline.
Unless your spotter is a qualified and experienced coach I would advise against it. It takes time to learn to spot this skill safely and correctly and its not safe to just gett anyone to do it.