Originally Posted by
jtav559
98.2% of the time (or close to it) when at least 1 passenger in a PNR has pre-check, any passengers without pre-check are grandfathered in.
Barring any behind the scenes TSA snafu / your in-laws are on some government watch list / etc - you should be fine with them receiving pre-check on their BP.
This is completely wrong in every way, and "grandfathered" is the wrong word here, even if it were true.