First of all, let me echo what everyone else said in that you are a wonderful person and neighbor for being there for her--if you're not already a "friend," then you should consider yourself one now, and that is something she needs rather she knows it or not.
Second of all, I thought Christy B's advice was really good--your friend is grieving--just started, maybe still in shock over her loss, but she needs to do this at her own pace, and in her own way. Having someone there for her when she needs someone is going to help her immensely.
I would add to that though, that going off her cues would be the best thing. Getting together some information about grief support groups is extremely helpful, but I don't think trying to force any of if on her is that helpful. It's almost like suggesting the grieving path she should take, and if she doesn't do it in the way that's natural to her, she won't appreciate anything anyone tried to "suggest" to her. Just have the info handy for her in case that's an avenue she wants to take. She obviously has some of her wits about her if she was able to get herself on medication, and also is demonstrating the will to get past it. Just *be* there for her.
You said you've prayed with her. If she's a church-goer, then offer to let her and her son go with you to church, keep offering to watch her son for her, keep offering to go on walks, take her where she needs to go, maybe drive her to the doc if she needs some support for that, but be a gentle encourager, not a pusher.
Now, on to her son and his eating. Like others have said, he's grieving also. I don't know if he was picky before, but maybe this is just one way he expresses it. For now, even though I don't suggest you make him a seperate meal, but on days when you know he's coming over, maybe ask him what he likes, so you can make it for everyone. Once or twice a week won't hurt anyone, and hopefully your family will understand that you are all helping someone through a loss, and this is one way you can pay kindness forward, by easing them through this initial time of their loss.
I hope I've helped some, but no matter what, just know that you're a very special person for caring for this woman and her son.
K. W