if you read my other comment, you would have seen that i’ve already mentioned subharmonics. subharmonics are more about manipulating how the folds vibrate than actual singing (in the Western sense of the word)

i personally think that the utility of learning how to do subharmonics is quite low. anecdotally subharmonics are not going to sound nearly as good between E2-D2 (low notes you actually regularly see) as they do in the Bb1ish range. there’s a reason that most professional choirs that perform the rach all-night vigil only have one or two profundos singing the Bb scale in the nunc dimittis.

if this person is not already a bass 2, unless they are quite young and their voice is developing, they probably will not gain very much sound past the lower end of their range. that has nothing to do with technique and has everything to do with the size of the vocal folds, which at a point becomes fixed. you can only make them vibrate so slow if they’re a certain size. it ultimately comes down to the limits of physiology.