RoshYeshiva,
let me start with a disclaimer that I am NOT an experienced shadchan.
That being said I think it would be very wrong to tell the girl’s side that he will forgo the shtreimel without first asking the boy’s side if that is indeed true. You may think the shidduch is perfect and that the 2 sides are complete fools to let the wearing of a shtreimel get in the way, but don’t try to play G-d and try to force the shidduch to happen. If he ends up not being willing to part with the shtreimel after they go out a few times and have feelings for each other, and if her side then breaks the shidduch because they were lied to, you will have caused a lot of needless pain and heart-ache.
If you think they have crazy hakpados you can try to reason with them but don’t try to lie to them to force them into something.