I think about discussing this kind of thing with a close friend, and I don't want to come off like I'm "against" my friend, or "for" the third party. Maybe I don't even know the third party (the DIL, for example)--I don't know if they're really trying to set boundaries or if they're really hostile or what. But one way I could approach it is to think about what would be good for my friend, regardless of the third party's motives. To take the subject of this thread--if my friend is saying, "She doesn't like me and I don't know why," one thing I could advise is that my friend just stop trying so hard to get "her" to like her. Back off for a while, let the other person initiate contact, and see what happens.
Like, Friend, I hate to see you putting so much effort into a relationship and not getting much back. Can you take a step back and lower that person's priority in your mind so you don't worry about them so much? Because it seems like that's what they've done to you. That way I would feel like I was really advocating for my friend's best interests, and not just obliquely trying to tell her she's being too pushy. Because really, I don't know if she's being too pushy--I know it sounds too pushy to me, but who knows what the third party is thinking? But if this person is really my friend, I would like to see her have less stress and frustration over the issue. (Of course, it's a bit trickier if she wants to see a grandchild more--you can't really say, "Make your grandchild less important to you.")
For example, my friend Amy. She is very generous in some ways and would love to buy birthday and Christmas presents for all her siblings-in-law, their SOs, and their kids. She started out doing this and quickly realized they were reciprocating only half-heartedly or not at all, which upset her. Yet she kept doing it, not because she truly wanted to, but because she thought eventually they would return the gesture (not to get gifts per se, but to reciprocate the social bond). Not only do they not return the social bond in some way, they often don't acknowledge/thank her for the gifts, and won't respond to questions about what kinds of gifts their kids would like, or how best to get the gifts to them.
Amy sees them as rude, ungrateful people who are trampling on her nice gesture. Maybe they are, I don't really know them. Maybe they are normal people who are silently screaming, "STOP SENDING US GIFTS! WE DON'T WANT TO EXCHANGE GIFTS WITH YOU!!" and Amy is trampling all over their boundaries. (In which case they should just be more direct about it, of course.) But it doesn't really matter to me. To Amy I say, "You get so frustrated about this, and these people just aren't worth it. Stop prioritizing them higher than they're prioritizing you. Save your time, money, and effort for people who will appreciate it, and just don't send them gifts anymore, and don't give it another thought. Because clearly they aren't."