The only trouble with reptile things (my family had everything ranging from annoles to bearded dragons to a savannah monitor ^~^;), is that they are not only sometimes not made for water, and can leech color, but are sometimes not smooth and can be jagged, which will hurt your betta.
If theyre smooth, then Id say go for it. If theyre non-toxic (should be), Id put it in water for a few days and see what it does.
Also, with the caves, check to see that your betta could EASILY get its entire body out, and that they could figure how to get out. Sometimes theyre not the brightest and wont be able to :c