I too have been dealing with Guzman Ariza & Asociados recently. As far as I am aware, `foreign' wills and testaments are recognised in the DR, but they are still bound by the laws of the DR. For example, if you hold real estate as an individual and not a corporation, your estate will be subject to the `forced heirship' laws of the DR whereby you have to apportion a certain percentage of your estate to your heirs (depending on how many you have....). You cannot just leave 100% of your estate to your partner as you would in the US or UK.
I am arriving from the UK shortly and shall be arranging for Guzman Ariza to draw me up a will as I don't currently have one drawn up at all. If I did already have one I too would ask Guzman Ariza to check over it for me to put my mind at rest.