Can I amend a prior years return to add an additional exemption if the other party also filed his return that year claiming himself, but he has since passed away?

It depends. You cannot amend to claim a deceased person who claimed his own exemption unless you met the requirements to claim him when he was alive. The following rules apply to claiming a dependent.

• Who can I claim as a dependent?

You should be prepared with all the documentation showing you were eligible to claim the dependent because the IRS will expect the burden to proof to be on you.  Two tax returns cannot claim the same dependent.  According to you, this person filed as independent.

Sending sympathies for your loss.