Daughter Has An Infant 3 Mos. Old. She Became Impacted

Daughter has an infant 3 mos. old. She became impacted the other day took her in and they put her on miralax and prune juice. She is feeding on prosobe. My daughter wants to mix in breast milk also. She is very gasy and wakes up every couple hours because she is hurting. Do you think that is a good idea?