The SfB Windows user get the following error message, when he try to initialize a screen sharing with a SfB MAC user:
Error: Screen presenting isn´t supported with this contact.
According to the following document here, this is currently not supported, however a workaround is mentioned:
Users can join a conference call or use "Meet Now."
A SfB MAC user can share his screen with another SfB MAC user or with an SfB Windows user. It affects currently only Windows SfB user, who wish to share his screen with SfB MAC user.