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 pronunciation 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 pronunciation or proper nouns, maybe.
¹ dieresis in Spanish are rather uncommon, to say the least.