I fully understand your concern but should be no problem la, coz the time for eating solid food is coming, I think your BB can digest well and gain weight very fast ga, don't be afraid.
My elder daughter was even worse than your friend, born with 6lb and got 11.5lb:-( :-( at 4 months and I was drove to crazy that time.
But remember to try feeding different solid food before 1, otherwise will have big problem on eating solid food when they grow older ga. Like me, I spend lot of time to make my elder daughter eat more solid food, and now she is the best amongst our cousins and always eat many food by herself (others need to be chased and feed and don't accept various kinds of food.)
Don't put pressure on yourself and try to enjoy the every moment with your baby la !