Whom would you consider in the story as a foil to the Capulet family nurse?A foil is a character that contrasts with another character and so...

Well, I think a foil can also be someone similar to the main character in certain ways, that is, in some key way, a reflection of them. Otherwise, for example, the Prince would be a good foil to the nurse - he's nothing like her, he's in control, unlike her, ... you get my point.

I think the best foil - and the most obvious "pair" with the Nurse - is Friar Laurence. Why?

Firstly, because the two of them are crucially the only understanding adults in the play: both of them are the generation above Romeo and Juliet. They are, moreover, the only two adults who are entrusted (by Romeo and Juliet) respectively with the information about their love and marriage.

Both of them provide advice which is then followed, and both of them actually interfere with the course of events: the Friar actually performs the marriage ceremony, and the Nurse carries messages between Romeo and Juliet.

And lastly, in the event of everything going wrong, both of them eventually resort to cowardly, backing-out-of-it advice. Here's the Nurse:

I think it best you married with the County.
O, he's a lovely gentleman!
Romeo's a dishclout to him.

And here's Friar Laurence:

Come, I'll dispose of thee
Among a sisterhood of holy nuns.
Stay not to question...

There's even a moment when Shakespeare mirrors the two characters explicitly:

NURSE:
Where is my lady's lord, where's Romeo?

FRIAR:
There on the ground, with his own tears made drunk.

NURSE:
O, he is even in my mistress’ case,
Just in her case!