Re: Similar vs. Similarly

>> "Similarly to the stone cameo of his wife Livia (fig. 2), this glass cameo
>> features an image of the head of Augustus in total profile."
>> 
>> I think it is correct as-is.  My fiance insists that I should be using the
>> adjective 'similar'.

Your fiancee is correct.  Similar is qualifying "this glass cameo".
If you rearrange the order of the phrases, you get:

"This glass cameo, similar to the stone cameo of his wife Livia,
features an image of the head of Augustus..."

which should make the meaning and grammar clearer.