generally, dried pea flakes are good for putting weight on (you can get them cheap from pampered piggies). also sunflower seeds and oat flakes.
If she has runny poo then I would not try and encourage her to have that extra amount of fruit, to be honest. You probably have experienced it yourself - too much fruit at once can make the runs worse! If she is on antibiotics she should get some probiotic as the baytril (guessing here?) will upset the tummy. BirdBeneBac is good, as is BioLapis. Make sure you leave about 2 hours in between giving the antibiotics and the probiotics.
If the weight is seriously down, give Critical Care (your vet might give you another brand, they usually arent that different) by syringe, mix some dextrose in with it, alot of piggies like it better that way which will make feeding easier for you, and also give them a bit of extra energy.