There are many names and family names in Spanish that contain the combination of vowels "oi" (Roilán, Moisés, Loinaz, etc.). When these names are pronounced in French, the original pronunciation is completely changed. Is there a way to modify the spelling so that I can describe the Spanish pronunciation to a French speaker?
If it is pronounced [oi] as I think, you just have to use a dieresis ¨ (tréma in French). It already exists in French, for instance my first name is Loïc, pronounced [loik], as opposed to Loic [lwak].
In French, this pronounciation often goes along with y in written text (I think of “oyez” and “royal”), you can use that if you want to describe it.
It's also the pronunciation of -ille in French, which forces the i out of any bigram it could form with the preceding vowel, as in “paille” (and arguably, in “corbeille”).
I have the feeling there's something about half-vowels missing in this answer, but I can't type what I don't know about.
As Evpok mentions, if you want to force the spelling to be well-pronounceable, use a dieresis on the i, but I'm not fond of this solution. You'll have a hard time explaining it's a Spanish name if you add this kind of punctuation to it¹. I'd just introduce it well-pronounced, or correct the first one mis-pronouncing it, and that'd be a good occasion to discuss foreign pronounciation or proper nouns, maybe.
¹ dieresis in Spanish are rather uncommon, to say the least.