That's what I'd do, trying to avoid the contrasty light ... but if the
guy is going to the rain forest and can't wait long for better light
then he might try some of these solutions with wider contrast ranges.

It's a lower contrast film (I like it for portraits) but it's still
tough to shoot slide film in contrasty light, I feel ... it should
perform better than Velvia under these conditions, but waiting for
lower contrast light is still better in my book, if he can wait.

Nope, Astia 100F is a pro film like Provia 100F and the 3 Velvias ... I
used to use Velvia (very saturated) for landscapes, Provia 100F
(somewhat saturated) for wildlife and Astia 100F (neutral saturation)
for portraits ... Fuji also sells Sensia, which is a consumer version
of Astia, which is probably what you're thinking about.  Actually
Sensia might be better for traveling to the rain forest since you won't
be able to keep the film cooled.

I don't think there's a consumer version of Provia 100F, but could be
wrong.  For sure it's not Astia though.

Maybe another half stop or 3/4 stop ... which is why it's still tough
shooting slide film in contrasty light :)  And why I mentioned print
film.

I was shooting macros in a forest on a cloudless day a couple of weeks
ago (with macros you can control the contrast with diffusers, as you
know) but I thought the light was way too contrasty to shoot the actual
forest with film.  Since it was kind of pretty in the small meadow with
the wind whipping the grass and aspen leaves I thought I'd play around
with digital and go with the contrast and do some abstract blurs,
shooting at slow shutter speeds and panning with the wind, then relying
on the RAW converter software to compress the tonal range to something
workable.  Here are a couple of results shot handheld at 1/4 - 1/2 sec,
the streaks of light are specular highlights from the direct sun
hitting leaves ... I don't think I could have done this with slide film
due to its tighter contrast range, the highlights would have burned out
too much or, had I exposed for highlights the dark areas would be too
dark ...

http://members.aol.com/hiltonfotography/forest_2_detail.jpg
http://members.aol.com/hiltonfotography/forest_1.jpg

Bill