I'm fine with encouraging girls to play with Legos. Plenty of girls enjoy playing with Legos, just typically not as much as boys do - and Legos really do help develop spatial skills.

But this isn't the way to do it. Hold some special Lego-building sessions for all the students, or make sure there are enough Lego bricks for everyone so there is no competition for pieces, but this is just trying to make little girls into little boys, as if being little girls with little girl interests isn't good enough. Her actions are harmful to both boys AND girls. (You can bet that the little boys are getting the message that authority figures are going to lie to them - so that is at least one good life lesson.)

She's also wrong about the quality of the Lego sets intended for girls. These sets are typically better designed (from a structural point of view) than the sets intended for boys, simply because the designers are not constrained by the need to mimic branded designs from movies or comics.