I was similar to what you mentioned with my mom when i left home. I wouldnt call every 5 minutes but i talked to my mom every day for about an hour, wrote her nice letters showing my appreciation and sent her pictures. Eventually after a few years it wore off. Now im that way with my daughter lo. But shes 4. Shes spending the summer with her dad and calls me 1-2 times every day and we talk for 30 minutes. Shes got a stack of pictures shes saving for me that shes colored and she picks flowers for me constantly.
There are many reasons for a child and parent to be close like that and often times parents wish their child was that close. Its not like the child refused to move out so she could be with her mom or anything.
Perhaps she realizes how short life is so she wants to spend more time with her mom, perhaps she feels bad for what she did as a teenager, perhaps the mom is now her best friend... its ok and normal, its not hurting anyone and its her child so i wouldnt recommend getting in the middle of it