If you request Nienna’s help she will be hard pressed to deny you. However she will typically not help unless she is asked to. She will offer comfort and guidance but to actually intervene in an altercation you would have to call on her. 

This is because the Valar do not meddle. They are there primarily for guidance and to keep the world in balance. Behind the scenes tweaking, if you will. Nienna is very gentile and will not break from her role which is to observe the world and it’s sorrows and comfort the souls in the Halls of Mandos —-

but she is also weak to those in need. If they request her help and she cannot see her involvement causing any major disruptions in the goings on of the world, she will most likely not be able to keep herself from helping.

Note the major disruptions in the goings on of the world bit. If Nienna were asked to take the ring she would not do it. Because the bulk of Middle Earth has not asked for her help. The Valar have not been called on to act, so they will not.

Now if it were something like driving away orcs, etc – she would oblige.