Yes, private diction lessons would be constructive. Also, most English-speaking choir members hardly have great diction, so she should know she isn't a minority here. If you ever do things in Latin or Spanish, she could probably model it better than anybody else, so you could take advantage of that. English is challenging for non-native speakers, because it is terribly inconsistent. I once worked with a terrific grade-school girl who was a fine musician and very smart. Her Latin and Spanish diction was flawless, but she had to be coached to let go of her rolled r's in English psalm singing. She can do it - it will just take some time and good instruction.

I once sang in a schola with a Japanese fellow, who sang heavily-umlauted U vowels. I just figured he'd been listening to a lot of early-music, French-Latin recordings. But I finally felt compelled to bring it to the attention of the director, who said, "He can't help it; he's Japanese." I said, "Oh? The person who taught me to sing Italian U was named Noriko Fujii."

Now, that only speaks to the possibilities, not the pastoral practice of dealing with volunteers. I'd be inclined to work on diction in general, without singling this singer out. If you do Latin, that rolled r might even be praiseworthy. Surely your choristers have other diction issues. Work on them all, in a positive manner, and he may come around.

I have similar problems b/c I'm in a church with various ethnicities. I just work with sections and try to be conscious of the vowel sounds in generalworking on a specific sound or tone, as Jeffrey mentioned. Occasionally, once someone has been with me for a while, I might single a person out. If it is done with a good nature and with love, people know you mean well and are happy for the help.

I have discovered, while teaching Polish seminarians and at the same time working in a Polish-American parish where I was required to learn how to properly pronounce the language for the Polish Masses, that it is far easier to figure out ways to teach those whose primary language is something other than English if you learn the vagaries of pronunciation in their language first.

For example, in Polish there is no vocalized "th", as in "the". It always comes out as a soft "d", because that's the way they've learned how to deal with a sound that doesn't exist in their own language.

What I have done with the seminarians is take them through the rudiments of consonant formations common to both languages first, so that they can get used to the idea of physically sensing what is going on inside their mouths and throats when they make certain sounds. For example, we deal with the hard "k" sound for the letter "c" (which in Polish is pronounced other ways), then the hard "g" sound (with it's slight shadow vowel, which I present as either "ih" or "uh") which they tend to "swallow" so it becomes rather gutteral. After they've started listening to the sounds and relating them to the sensations in the mouth and throat, I move onto the consonants and vowels that are not common to their language.

Also, try using a hand mirror so that the singer can watch their own mouth.

Salieri - try giving them a phonetic spelling of the word. When I worked at a Polish parish I quickly discovered that they always pronounce the "g" hard regardless the vowel that follows, because this is the only way a "g" is pronounced in the Polish language. I had to live with a hard "g" in "Asperges" and at the time I resigned myself to it. But as I write this, I realized that if I spelled it out for them as "an jus" (in two syllables), they would naturally pronounce it correctly (as nonsense words) because the Polish eye sees a "j" and pronounces it as an English "y".

YMMV.

Meanwhile, keep your stick on the ice. We're all in this together.

Will never forget the wonderful visiting Asian singer doing "The Holy City." You guessed it. "Jelusarem, Jelusarem, rift up ...."