She would be better off donating them to a museum or something like that and taking a tax write off. The write off's allowed will be much more than they would sell for.
Might check with Cabela's, Sportsman's Warehouse or some other
sporting goods store to see what they would offer, but it is not as much as you would think. Lucky to get the taxidermy bill out of smaller heads. If they were book heads, that would be different. They sell them on ebay once in awhile might check that out also. I would think she would be very lucky to get $1000- $1500 for them.