I'm assuming this is topical Minoxidil 5% used for hair re-growth. I'm also assuming that this exposure was a small amount and not an entire bottle exposed to the eye. If these assumptions are wrong, please inform me.
In this case, what should be the two primary concerns are his systemic symptoms and his vision.
Minoxidil causes a sympathetic nervous system ("fight or flight") to occur in the body and so topically, in the eye, it will cause the pupils to dilate. Significant pain and significant vision changes should prompt a visit to the ER today, or to his ophthalmologist if he/she is available. In the absence of this, copious irrigation to the eye with water should continue--you can't do it too much.
If after irrigating for 15-30 minutes if he still has no significant eye pain, vision problems or areas of obvious visual concern for damage to the eye, waiting until tomorrow to see his ophthalmologist is a reasonable thing to consider.
Because this drug has powerful systemic effects, if he experiences chest pain, shortness of breath, dizziness, or loss of consciousness, he should be seen at the ER immediately (this is likely a no-brainer to you, but bears stating).
Does this address your concerns?
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Disclaimer: My opinion is solely informative and does not constitute an informed medical opinion or recommendation. For an informed medical opinion and/or recommendation you must see an eye doctor.