With a poly coating on there it’s more difficult to say.
Personally I keep an eye on the colour of the wood. When it starts to look dry and pale, it’s time.
The wood will let you know, it will stop soaking the oil and wax and you then wipe off the excess and start buffing.
It may let oil seep out for a few days after, as it adjusts. Just wipe that away. Food grade mineral oil (which I use for all my boards) is non toxic, but it is a laxative, so do wipe up and excess before using the board 😁
Bowing and warping is a natural response to moisture changes. It may be unavoidable.
Getting the moisture content back to where it was may reduce the severity of it. Keeping it there means regular drying, oiling and waxing the board.
You can use things like bow ties to help hold the seam together. Blacktail Studio on YouTube has a good (imo) video on installing a bow tie.
For filling it, you can use a food-contact rated glue, like Titebond 3 and clamp the crack back together.
If the crack is too large to close, you can fill it with sanding dust mixed with a food safe wood glue.
Or! Go all in, remove some material to give you clean flat surfaces to glue in some new material. You can hide it or highlight it, depending on the wood you use.
If you are open to friendship with this person, then you could still go for a drink. It can be nice to have someone who has experienced similar negative things to talk to.
Ok