start with onelook--(onelook.com)
you enter a word in one looks 'search' and it will bring back all the dictionaries (on line) in which i found the word.

some dictionaries are better than other, and will list related words, or a I(ndo)E(uropean) root word, or even a
P(roto)IE word..

follow the links to the root,and find lots of related words.

(there are several post here that have done just that )

the American Heritage dictionary is one dictionary that will often give a IE or PIE root--but its not the only one.
(she could also go to the library and do this with the oxford (un abridged version) dictionary (as i did back in the 1970' and earlier!)

reprehender is most likely not a pure root, re/ pre/ are most likely prefixes, and er a suffix. the root is likely to be some form of hend

try looking up sugar, (from the french sucre, but the french got it from the persians.. )
related words include:
shingle (like a roof shingle)
crocadile
searsucker (the fabric)

there is a difference between a related word, and a root word. sugar and sucre are closely related, but but the french word is not the root word, you need to go futher back.

and many, many words are built up from 2 or more root words. (television, telescope, teleport, telephone, all share tele as one root, (but microscope, and phonograph are words that share the other half root (and there is no relation to microscop and phonograph, but phonograph is related to telephone)

and send her here if she get hooked on the process, we are a bunch of word nerds who love doing this sort of thing!

my other obsession