never heard that before !! babies, once they can grab stuff, and work out they can get it in their mouths, will do so .
more reliable signs of readiness for food:
loss of tongue thrust reflex
ability to sit unaided and hold head really well
pincer grip / ability to pick up food, chew and swallow
babies watch everything, they ry to chew everything,. but milk is the main source of nutrition , and the safest for the first 6 months at least and food is for fun until you are one!
baby books are not always up to date or even correct on all things.
there is little or nothing to be gained from early weaning.
if your friend's baby is showing other signs, like those above and the ones you have mentioned too, then she could try him with some finger food
waking up more frequenlty, wanting more milk, and doubling birth weight are not reliable indicators of pyhsical readiness for food