My son received a check for a wedding gift. Unfortunately it went through the washing/drying cycles and can't be deposited. How does he word a thank you note?

He calls up the person who sent it, explain what happened, and normally they giver will merely send them another one

He can't send a thank you note, until he has the gift.

The giver will notice that the cheque wasn't cashed.

If it is someone he knows very well, do as Chuck said, and have him call the person and explain what happened. If he does not know the person very well, I would enclose the crumpled up cheque so they know it didn't fall into the wrong hands, or that he is saying it got messed up in the wash, and maybe it really got lost, and he doesn't want to say he was careless.

The very least he should do, is talk to him in person, explain what happened, and what he did with what was left of the cheque.

I agree with both. We all make mistakes & sometimes owning up to them is hard but, if they cared enough to send him something as a wedding gift they should understand mistakes happen & will send a new check.