He didn't ask his mother to be there, he rang up to update her and vent a bit. If she was an invited visitor she wouldn't have needed to use her pass to access a restricted area. The mother, who is the patient, has the right to say who is going to be near her in the immediate aftermath of giving birth when she is in pain, bleeding and vulnerable.
As a result of the MIL's actions she also overheard very private and sensitive medical information that has caused the DIL considerable distress. I'm shocked at how many posters on here seem to think that is just fine. Apparently if someone gives birth to your grandchild they become incubators and lose the right to privacy, medical confidentiality and any consideration of their wishes at all.