If you had enlarged an opening on the inside of a concrete block wall (plus the cement board over it) to repair a plumbing issue, such as in my case to reach deeper into the wall cavity to get to a copper drain where it is nice and round, once you are done what do you do to seal off the void to first, keep out bugs and second, to provide more backing support for the piece of tile covering?

A friend said to just use those foam sprayer it seals everything and be done.

I thought about it and I think if it's PVC I would, but since it is copper, whoever next to come after me, fir whatever reason, might have to use a torch, and these foam, looks flammable to me and burying these copper elbows and couplings with these foam sounds like an accident waiting to happen ESPECIALLY if I do the tile so nice they have no reason to suspect there is foam and if they heat it fire aside could also be toxic fumes right?

So what so you do pack it with thinset or stucco?

You need some kind of backer for whatever you pack in the sum, or it will just keep falling in.If it's 1/2" concrete board, I'd probably cut a new piece.

If you do a good tile job, there will not be any air to support combustion, and the tile will block any fumes.