In a recent combat situation, a Yuan-ti cast Suggestion on one of my players, a cleric, and ordered him to run away from combat. Ingeniously, another one of my players, a monk, made an opportunity attack against the cleric as he left melee range. Because the monk has the Sentinel feat, he reduced the cleric's speed to 0 and prevented him from leaving combat.
My question is whether an opportunity attack can be made against an ally. RAW, an opportunity attack can only be made against a "hostile" creature:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach.
Would an opportunity attack against an ally compelled by Suggestion be valid in this case?