A fríend of mine has a custom 300WSM, after 2500 rounds, the gunsmith told him the rifle have an excess of headspace of 0.15 mm. Is it dangerous for shooting?
Must he rebarrel the rifle?

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Well, that's .0059 inch over the GO so, it's probably fine to shoot. SAMMI shows a maximum of .010 inch or, .254mm which should be a military FIELD. Because of the large, sharp shoulder tolerances have to be increased to make sure that Bills ammo works with Freds guns and Nancys guns work with Freds ammo. And then those pesky Europeans who can't transpose English measurements into grenouille and vise versa get involved and things get really messy.

Excess compared to what? Realize that the difference between a maximum chamber and a minimum cartridge is .013; you will see that .0059 is nothing. Sounds like that gunsmith just wants to sell a barrel.
.0059 is well within the normal tolerance for chambers and ammo.
If someone is worried about that, they can fire form the brass and not resize it fully. Or set the barrel back so that ammo is slightly hard to chamber. Or just shoot it; this is a non issue.

Sorry to hijack the thread, but this question has always puzzled me. On a modern rifle, how does headspace increase? Does the shoulder of the chamber move forward or does the bolt face move rearward?

Long to begin with? I would not worry I just bump the shoulder anyway.

More likely the brass is shortened by the sizer. I need to write a book entitled; "the myths of headspace". Of which there are many. Again,, .005 is nothing, well within allowable tolerances.
If it did happen, how does it in the rifle? It could be a number of things; including what you mention, all of which are quite rare.

ah yes - headspace - something that is lacking in mine

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99% of the time, guns develop excessive headspace because closet gunsmiths figure that they can substitute a cartridge case and masking tape for a headspace gauge or a ball bearing and a depth mic.

Sorry I'm sort of an asshole.