Make sure the "nipple" in the center of the torque converter has
absolutely no rust on it.   Test fit the flex plate when everything
is out of the car...the flex plate should go on easy over the nipple.

Use a wire wheel on the nipple if you have to.

If it is too tight a fit, the flex plate may not seat correctly when
you tighten it down, this puts extra stress on the flex plate and
could make it more susceptible to breakage.

Replacing the torque converter is also a good idea at this point if
she has broken two flex plates already.

-Paul