"His friend Samia is rich."
Translation:صَديقَتهُ سامْية غَنِيّة.
Just adding a note of Abdul Haq's. Hope I understand your questions correctly.
(1) "is the diacritic pronounced like a short of dipthong ie. sadeeq-a-oo"
If the noun for "friend" is masc., it will be صَدِيْقُهُ SadiiqUhu (not SadiiqAhu or Sadiiqauu) as the phrase above is in the nominative case - if we follow Standard. So, it is not a sort of dipthong.
(2) "does it essentially sound ... ?"
It is how Duolingo's teaching style is. They try to omit almost all the ending sound/diacritics because they want to apply some colloquial speaking whereby many ending sounds are not pronounced.
But, in Standard, the ending sounds are essential and should be written as they will determine the cases clearly, ie. especially for the beginners. To be noted that, for Standard advanced learners themselves, diacritics are not so important because they know how to pronounce the sentences without the help of diacritics.