How to save these preserved mangoes?

My friend washed some mangoes, peeled the skin, and soaked them in a solution of salt and water. My friend then washed the mangoes a few times to remove as much salt as possible. My friend then exposed the mangoes to ozone under water, probably a few times. My friend then soaked these mangoes under a solution with some preservatives and other substances. He then pack these mangoes into big plastic bags, tried to squeeze the plastic bags to remove as much air as possible, and then tie up those bags with zip-ties. There were two plastic bags covering each batch of mangoes and they were stuffed into plastic buckets covered with their plastic covers. After a few weeks, bubbles started to form in the solutions, the plastic bags swell with gas, some buckets swelled, and some solutions poured out of some of the buckets. I assume it's due to fermentation. May I know how can my friend stop this?

I know two ways to stop fermentation. First is to use sulfur dioxide and this can be hard to figure out. I have little experience using sulfur dioxide. The other way is to clean and use a sanitizer on all equipment. Then boil the mangoes for a few minutes. And repackage the mangoes in clean and sanitized packages.