Many countries have a web site that tells people who (and who doesn't ) needs a visa. I remember looking at Estonia's last year before we went to Tallin.
Your frind should check each country that the ship will visit. If she's currently in the US and will be returning to the US, she she make sure her immigration status is secure so that she doesn't have trouble coming back in.
We loved our Constellation Baltic cruise. Have a great time.