hello
i think it would be a good idea as long as you're
sure Nana will accept another rabbit. you may want to get a young male. I only say this because females are so territorial... had she and Baby been together forever or were they introduced when they were older? perhaps Nana is just a very loving and nurturing bunny who will accept any other bunny companions- if this is the case then i think you should get her a friend. it could help her to forget the loss of Baby and not get lonesome.
however- if you think there is any chance she might not accept a new bunny friend then i would hold off on getting a new one. it may end up stressing her even more if she has already lost her Baby and then a new rabbit comes into her territory.
this is just my opinion, if you're sure Nana will accept a new bunny than i would go for it it could help her heal from the loss of Baby, but if you are not absolutely sure of that then I would wait a while
good luck with whatever you decide to do!

hopefully somone on here has experienced something like this and give you more definate advice.