Ideas change all the time. I remember in the 1970s, a paediatrician advised me to hand the baby round as often as possible right from the start, preferably leaving the room myself for a while myself "so Baby doesn't fixate on you as his only source of care and comfort". His belief was that, the more often (and earlier) children meet new people, the easier it'll be for them as they grow up. Back then, we believed that we'd naturally bond with our babies - nowadays, apparently we should be working at it by isolating ourselves from other people for a specific timespan. And soon some other "craze" will take off. Unless there's actually child cruelty involved, I can't see any harm in these changing points of view.
My mother was a real pain in the neck about advising me how to care for my baby (to be fair, she probably felt the same about me). On one memorable occasion, she actually shaped up to punch me because we held different views on the best way to clean a baby's bum! Because I stuck to my guns, she would bring it up (preferably in company) for years afterwards, as an illustration of what a terrible mother I was. I wouldn't wish that on you, Glam, so I think your best bet is to bite your tongue and let her bring up her baby however she thinks is best. As time goes on, those 3 months will just be a hazy memory.