I think your grafted plants have been pampered in a 'cactus factory' (not nursery!) in colder climate before you bought them. Therefore they cannot cope with your climate. And in general plants/scions lacking of chlorophyl are very susceptible to heat and too much light.
The monstrous cereus would do much better on its own roots. I would cut off the understock, apply some rooting hormone to the cut if you have and keep it vertically in dry soil, coarse sand or pumice IN your home for 2-3 weeks, then keep it slightly moist or spray it and wait till it is starting to grow, then slowly acclimate it to its future stand.

Jordi

thank you jordi and by the way do you notice any rot in the roots ?

Yes of course, the understock of the yellow peanut cactus has rotten roots.
Looks like too much water and a soil mix too high in organic material.
Also the Mamm wildii in the background seems to get too much water and nutrition. It looks a little leggy/etiolated. More light and less water would be good!
Jordi

could he not cut off all the dead and let it callous over and try it again...hey It might save it I have done this thinking its a goner and just lay it to the side and then it started to revive....so who knows what cacti will do. But I would try it.