I am not an expert in Irish placenames,just someone with an interest who can speak Irish.
I would take the "sweet valley" meaning with a pince of salt. I would say this is influenced by Avondale which is near Avoca and which COULD be translated as lovely valley from the irish for delightful "aoibhinn"(pro. EE-ven). However as far as I know Avondale is named from the Rivers Avonmore and Avonbeg(i.e Abhainnmór and Abhainnbeg-the big river and the little river.)
Irish placenames are often composed of similiar words,which are English versions of Irish words;for example Dun(fort),Bally(town),Glen(valley),Loch/Lough(lake).So for example Glendalough means Valley of the two lakes. So you can look at many placenames and take a guess at the original name in Irish.
The meaning of Avoca is generally taken to be "the meeting of the waters",widely known because of the famous song by Thomas
Moore. In the Vale of Avoca 4 rivers meet and then run into the sea.
Now,áth usually means a ford when used in placenames. The Irish for an estuary is inbhear(inver). Could Ath Inbhear become Avoca? I was not totally convinced. Another,theory could be that one of the Irish words for ocean is bóchna.Maybe Avoca comes from Ath Bhóchna(aw vocna); I am not sure whether bochna is a modern Irish word or not though.
I looked up the modern Irish name used for Avoca and it is Abhóca,which sounds like a direct modern translation of Avoca into Irish(as opposed to Irish translated into English) This would indicate that maybe the usage of Avoca for the town name is modern,influenced by the famous song about Avoca.
I did find out that Avoca town was previously called Ovoca and that before that it was called Newbrigde(An Droichead Nua). One source claims that Oboka was what Ptolemy
called the town. http://www.n-ireland.co.uk/ubb/Forum10/HTML/000193.html
So... you could tell your cousin that her name means "meeting of the waters" or "mingling of the waters" but not beautiful valley.Hope