Never... I completely agree with your decision. I think that as J's grandmother, what she is doing isn't really a "gift" anyway... and we all need a step up once in awhile, right?
I figure that if she wants to do this for Lent, let her. If she's really not doing it out of the kindness of her heart, that is her deal. If you can somehow find a way for this to benefit your son, her grandson - I don't see anything wrong with doing it.
Also, if things go sour or she "throws it in your face" at some point in the future - well, can't you decide to pay her back then? At that point, if you get there, you won't have to worry about whether or not her intentions are good or not anymore, or worry about potentially "hurting" someone who is trying to do a good thing, because she'll have shown her colors are still the same. In my view, it is a situation that could still be "rectified" if it doesn't work out... right?
And as for the child support situation... it has nothing to do with that - whether she wants it to or not. And there are lots of things that can happen between you filing whatever instruments you need to file and him actually being hauled off to jail. I figure they will probably find him in contempt - that's how he ends up in jail, by violating the Court's order. But he will have a chance to pony up before he gets stuck in a cell, and that's what he should be doing anyway, paying you.