It woprked for us, even as a meatball replacement for missing window glazing putty out there.
Make sure it is a removable caulk ("cawk") so it zips out easy when he wants it to.
If he doesn't fancy working with cawk guns, Mortite comes in gray or brown. It's affordable, and it comes on a ropy roll that can be made to fit all cracks. (Get the big roll.)
The gray one sticks better than the brown one. We use that every year too, on windows we might want to open if there is a kitchen fire with smoke to clear in the winter.
Mortite is a lot of fun to work with and surpasses its label's claims.
One room here still has the pieces I used years ago to tack up chart paper for a class, that Hardi (I think) turned into little tiny gargoids. Now we stick other charts to it. I should carry it in my gigbag to stick music down on windy days..... pull a little piece down and stick it in my pick bag.