I tend to agree with Daemonax. Perhaps a flash player is included by default
to give a better "out of the box" experience, and Gnash is probably the
better choice since development on Swfdec has stopped. However, I have the
feeling most Trisquel users would know how to install either package using
Synaptic. Trisquel users likely install their own system and should have
some knowledge of the package manager. Searching for "flash" in Synaptic
pulls up both Gnash and Swfdec.
Should the user have someone install Trisquel for them, they can ask the
person installing the system, check the section in the Documentation section
of the website which explains using either Gnash or Swfdec as alternatives to
Adobe's flash player, or ask in the forum.
Personal note: I would hope that anyone installing an OS for someone else
would spend some time going through the basics, which in GNU/Linus definitely
includes the package manager.