Cola etc is like pH really below 3, and then you taste the acid. Better to use it to get between 3 and 4 before adding material, and it jumps to 5 at the end of soak or brew. You really can't taste it then anymore. This way he sees no danger, especially since we talk about little cups here, not buckets of cola + popcorn "cinema-style". Probably your warning of negative impact of phos-acid must be linked to quantities?
But instead of starting with a certain acidity point and go with that all the way, one can also work intermittent, making measures in between, and correct tiny:
Lately, my friend brewed vine and acacia and kept adding drops of phos-acid till pH touched 5, boil for some time and pH went to 6.8, adding again to 5, boil some time, again 6.8, back to 5 etc. Like keep pressing it down to 5, the final ended like 6.
At some point, one sees that the pH does not tend to rise anymore as fast as in the beginning, stagnating, this indicates some degree of material exhausting and/or saturation of the wash, indicating time for a new wash batch. Not sure if "saturation" really exist, it just feels that way.
Very hardly reducing volume to one sip, he wasn't even able to physically concentrate the remaining phos-acid to any taste-able amount. Just know what you're dealing about since you can't boil/vaporize phos-acid off like vinegar, treat it carefully, one is really fine.
The taste of an acetic-ized brew is just horrible compared to system described above.
Some people might find this pH jockey game too much of a hassle, but he loves to "feel" and play with the brew as it goes. He really thinks/experiences it pays off. Maybe he changes mind later, but not for now.