Treasurer is using original savings/checking account for our 501c3 but she is the only authorized user on them. Is this legal? Bank is only in the state we incorporated in although we are international org. and it offers horrible interest. She is interim treasurer till October 2014.

Your question seems to actually be two-fold: first, is it legal for only one person to have authority to make transactions in the account, and second, who has authority to decide where funds are invested. The first depends on what the bylaws say - in most organizations the President typically has authority as well as teh Treasurer, and I would say it's good practice in case something happens to the Treasurer. As for deciding where to have accounts - that would typically be a matter that the Treasurer or Finance committee (if you have one) to bring to the Board for approval.

I will agree, first it is legal, but really a stupid idea. In case of her death or if they resign, there should be safeguards in place. But that choice is up to the group and how they established the rules for the account.

As for the second part, does not the board decide where to put the money, the treasurer would normally only recommend but the board would decide where the money was to be put