Odd behaviour?

I have a male and two female Bettas living in one of my tanks with some Guppies. The male and one of the females are thriving but the other was very small when I got her and has remained small. All three are very peaceful and there has never been any sign of bullying in the tank - it is very thickly planted with lots of rocks/caves. Unfortunately one of the Guppies had an anchor worm and so I treated the whole tank and the smaller female Betta is looking very poorly indeed now - so much so that I suspect she is going to die. It is really sad to see her but the other two Bettas keep going up to her and settling near her for a while in turns. They don't touch her but swim off after a while. To date they have not taken any notice of her particularly and I can't decide whether they are being sympathetic, curious or waiting to eat her when she is dead. Is this normal fish/Betta behaviour? I am reluctant to stress her further by transferring her to another tank on her own as I am fairly certain she is dying, but not sure what to do about them or if I need to.