As an avocational paleontologist, this one yanked my chain.
If it's not a fossil, why perpetuate the idea that it IS one and why
add to it the fabrication that it is valuable and rare?
Ideally, my thought is that your friend should tell her boyfriend
what you told her. The longer she keeps the secret the further the
repercussions will reach. If she can't or won't tell him, the next
best suggestion (and a small prevarication) would be to have her
suggest they take it to a nearby natural history museum or a college
with a geology department and have a professional tell them what kind
of "fossil" it is! She can say she's curious to know more about it
(remember, provenance is what makes an antique more valuable). If
they make the visit together, they can have a good laugh together
about being fooled. Then, the "fossil" could be gracefully retired to
a jewelry box or even placed in a display case as a private joke
The museum where I've volunteered for years gets visits like this
all the time - it's no big deal, though the people who visit
sometimes feel a little foolish. People bring in "dinosaur eggs" or
"dinosaur bones" that are just unusually shaped rocks. The curator of
paleontology at the museum makes a visit several times a year because
someone found a "dinosaur" when they were digging their pool. Once in
a while, it might be something like a mammoth or a fossil turtle, but
it usually turns out to be old sheep or cow bones or even a healthy
Honesty IS the best policy!