Mother is shocked by something ‘disgusting’ her mother-in-law did to her baby

What would you do?

A mother has reached out to other parents for advice about something her mother-in-law did to her baby. She explained that she witnessed her mother-in-law sucking on her baby's hand. To give context, she said that her baby is going through a phase of sucking on his hand, so the mother-in-law decided to put his whole hand in her mouth as it would be good for his immunity. The mother of the baby was shocked and has called the act 'disgusting' and 'disrespectful'. She's asking if this has happened to anyone else and also if an immunity expert could confirm if it is, in fact, good for the baby's immune system. In the post on Mumsnet, she said:

"My 4-month-old baby is going through a phase of constantly sucking on his hands and fingers. Yesterday MIL said to baby ‘you like having your fingers sucked don’t you?’ and proceeded to put her mouth around his entire hand and suck. My involuntary facial expression was shock. Seeing my reaction, MIL said in a baby voice ‘it’s good for my immunity mummy’ and then turned to the baby and in baby talk said ‘you need to build up your resistance. That’s a big word, resistance’ and so forth. ...She then continued to suck on his hands and fingers in front of me.... AIBU or is her behaviour as obnoxious as I feel it was? I don’t mind people touching or kissing the baby’s hands... but full on coating them in saliva just seems disgusting. And odd. And bloody disrespectful to carry on with if you know the mother isn’t ok with it. But I’m a first time pedantic mother so happy to be told I am wrong and overreacting. Any immunity experts? Any MIL advice?"