I have only had kids come the week before for a couple of hours. I had one child come for three days the previous week for a couple hours each time before starting full time. What that mother is suggesting is way overboard and not needed. It may be detrimental to the child transitioning since he may sense the mother's tension and whatnot.
I would tell the mom she is welcome to come for one hour on Monday and then she has to leave as it is not fair to the other children that your new dck gets to have his mommy here to play and they don't.
Good luck.