Rainier is usually the one playing distraction, being a solid wall of person, but he's also damn good at following orders given and had no intention of protesting however Rey wanted to do it.

She's a good distraction, and Rainier moves faster and quieter than one would expect from such heavy armor. Flanking the beast isn't a difficult task, and he only waits until he's in position before moving to attack. He has no intention of letting her fight on her own for longer that humanly possible.