I'm not at all sure I agree with that. The law is against "invading" someone's mind - that doesn't sound like a prohibition on mind magic done with consent. LtW's efforts to help heal the damage Peabody did to the younger personnel would fall under a similar situation. Also, for that matter, Molly and Harry's sparring sessions about trying to plant an idea in each other's head, which they apparently did a considerable amount of, would also be prohibited under the interpretation that consent doesn't negate the violation of the 'invade' law.
Almost all of the Laws have a grey area around them, where some Wizards are allowed and others are not, sometimes officially (the Wardens and the Gatekeeper, for ex, can study Outsider knowledge a little more than others, from necessity), sometimes unofficial (politics and custom).
Planting an idea in someone else's head, with permission, doesn't violate the Third Law because it doesn't tell you anything about what they're thinking or knowing, but it presses against the edges of the Fourth Law. In that case it's probably just this side of the line because nobody's really being 'forced' to do anything or think anything...quite. But it
is right at the edge.
Some types of illusion magic are mentioned to press against the edges of the Fourth Law, while remaining technically legal. The Council specifically allows the use of compulsory 'sleep' as a healing tool, though I suspect they expect it to be done sparingly and only with serious reason.
But 'permission' only goes so far. Helping someone suicide with magic, for ex, would be 'killing with permission', but to do so directly would surely violate the First Law. For that matter, helping Harry do what he did memory-wise means that Molly was at the edge of violating the First Law as well as actually violating the Fourth.
But those grey areas are just that, grey areas. Some people may be allowed to get away with entering them where others are not. For ex, remember that Charity told Harry that Gregory had them practicing magic that was at, as she put it, 'the crumbling edges' of the Laws. Not quite violating them, but close enough that the Wardens might take some kind of steps even so. A full Council member might be allowed to get closer to that 'crumbling edge' than a minor practitioner...or might sometimes be expected to stay further back. Politics is a fact of life.