DC1 has just finished school and is off to Uni next Sept. He has an excellent gap year opportunity paying in excess of £20k PA. I’ve asked him for monthly rent of £250pcm which will include everything he enjoys now. He and I have discussed it at length and think that’s fair. He doesn’t know, but I’m planning on saving at least half of it for when he starts Uni, plus it will teach him the value of budgeting (and not frittering away your first full time wage on crap which his dear mother did!) 
Enter my dear ex—MIL. So as not to drip feed, my DCs dad died when my youngest was a baby. I’ve maintained contact with his family as was their dads wish, even though they disapprove loudly of everything I do or have done in the past. Now the DCs are adults I’ve gone ultra LC with them but the kids stay in touch regularly. Anyway, DC told me this morning that ex-MIL is horrified that I’m making her darling grandchild pay rent and has offered to “make up the shortfall” in his wages! I’m livid…even though my DC are adults I’m being undermined. My thoughts are that this is going to teach him nothing about the adult world and he’s just going to carry on running to his GM for things.
WWYD?
a) persuade DC to decline the offer?
b) Tackle ex-MIL yourself?
c) do nothing, DC is an adult and can make their own decisions.