Boiling juice almost always changes its flavor, and not typically for the better. I'm not sure how much pectin is in mangoes, but it will set a pectin haze as well. I wouldn't want to boil the juice, and I honestly don't think there's a need.
I was worried about sanitizing the fruit and habanero before adding it to the secondary. What I plan to do then is cut up a habanero and let it sit in vodka for an hour. then put it in the secondary. With the mangoes, I was going to cube them up, put them in the freezer for at least a day. Then , using a juicer on the cubes to get the juice. Then add the juice to the secondary. rack the beer ontop of the juice. Then added the habanero 3 days or so before bottling
why not make your juice, boil for sanitation, then cool covered and add the juice to your secondary? Seems to me you are getting further from being sanitary while complicating your day. Not that your day matters much to me, but that's not going to get you a sanitary juice in the end.
Pasteurization is the route to take
180F then immediately cool
or I think you can do something crazy like 135F/30mins which would save him A CRAP LOAD of time vs a day.
Although taking the cubed, frozen, juiced mangos pitched directly into the secondary is better than the negatives of caramelized flavors from his juice? He wasn't proposing a sanitary process of putting mango juice in his hard earned product, why spoil it now, or take the risk? Nope lets worry about pectin haze... ? I don't get it.