I'm with the others. I wouldn't bring it up. If she does happen to talk to you about then it's totally fine to suggest maybe she could take him to a doctor.

My vet, she probably has taken him to the doctor many times. Lots of things can make kids snotty like that, dairy, allergies etc. its not uncommon for kids to be that snotty, I actually know a few myself. I certainly don't envy the mums having to deal with that much snot.