Any advice on inking a ‘flabby’ person…

You will have to do a tight three point stretch, may need to push slightly down on the skin (a bit more than normal)

Also Depending on where it is on the body you could get them to position them selves in a way it taunts the skin. (lower back: a slim person could sit in a 90o angle and have taunt skin but a heavier person may need to sit leaning more forward to pull it.
excessive skin moves very easily so when applying a stencil get them to rest the area comfortably (bingo wings have a mind of their own so some times best to get the person to put arm up in the air and then rest it by their side just to allow skin to rest comfortable and it wont be dragged forwards or backwards)