The mother may want to visit the grave very frequently. At the grave, she can care for and be close to the only physical part of your child she has left. Allow her to visit as often as she needs to. You do not have to go with her, however, don't discourage her or tell her it is unhealthy. This may be detrimental to the lines of communication.
Many mothers benefit from reading books on grief.
Buy her a few books to share together. She knows in her heart that her life and the life of her family will never be the same. Acknowledge her pain, respect her feelings of deep loss and try not to rush her healing or offer a quick fix it for her grief. She will accept the tragedy over time, in her own time. Don't insinuate that she needs professional intervention because she has a desire to talk about your child.