One of my friend is getting on a scientific research about some guitar bodies and their mathematical effects on sound and tone.
this subject has come up in the past, in 2002 and 2006. There is a four page thread.
I posted some links to research in the area of guitar dynamics. Have your friend take a look at those. I checked and most of the links are still valid.
I get the feel from your questions, however, that he's focusing too much on the (mass) properties of wood, while ignoring some fundamentals.
Wood simply varies too much from blank to blank, from tree to tree, from species to species.
Furthermore Jem bodies are built of three to five pieces, which in theory could each come from different trees or at least different parts of a tree, adding variation.
Note that most wood properties have standard deviations in the order of 20%!
Not that it matters, since the overall resonant properties of the guitar will depend less on the wood than on the shape.
As specific mass goes up so does stiffness; and it is the ratio that determines the frequency.
So if, for instance, the spec. mass is 0,3 kg/L and the stiffness is 6GPa, at 0,4 kg/L the stiffness might be 8GPa. Even if it goes up disproportionately (+20% mass -> +10% stiffness) the ratio would go up by 10%, and the resonant frequency would drop 5%.
What is more important is the stiffness ratio parallel/perpendicular to the grain, the overall shape, the neck-joint, the trussrod. All of these of entities are completely independent of the mass of the basswood.
If your friend is serious about the science, he will also be better off if he gets his data from proper sources, such as the wood suppliers themselves or institues that test wood such as the US forestry labs:
or civil engineering institutes.
The FPL has data on Tilia (incl. American Basswood):
as well as European basswood (Linde)
which appears to be 40% (!) stronger (no details on stiffness or mass).
Full pdf on Tilia:
Your friend should build a variational model, which would allow him to analyse different classes of woods with different properties.
Then he could validate the model by doing experiments and measurements, such as the ones linked above.