I used to run a peer rope group out of my house until I moved for work. Box ties come in two flavours if you want to get technical. Wrist to wrist, and wrist to elbow. Wrist to elbow is preferable but not always possible if the model isn't flexible enough. I'm a struggler and I've had difficulties with them. You need to make sure the knot isn't actually on the elbow nor wrist.
Wrist to wrist ties are a lot easier to get out, I'd hazard a guess that your rope buddy has dexterous hands. I have skinny wrists and therefore it's pretty hard to stop me from rotating them. I found the hardest ones to shake were where they were pulled up towards my upper back with ropes over the shoulders or down with a crotch rope. If you tie them without those anchors it is almost laughably easy to wiggle out.
Full disclosure my main goal with my rope group was suspensions and therefore all the chest harnesses we practiced were for such tasks. There really can't be much movement in the arms as the point of a boxtie is to make the upper body function as one piece to allow for safer distribution of weight when under load.
I always got out of slack ones by "chicken winging" ie. Pulling my elbows towards each other by jutting out my chest and then releasing quickly so the rope sort of hops down the arm giving slack. My friends stopped this by only really tying my wrists to the opposite elbow and pulling them as close together as they could comfortably. I normally had my hands poking out from either side of my body so there wasn't any knots to reach on the first place.
I have been in the other side of the rope too. Sometimes you just have to make up a slightly different style for each model. Larger or less flexible people can actually be harder to tie because the body is able to compress over time.and give slack. It sounds like a cop out but there is no real one size fits all boxtie. What works in restraining me might not work for your rope buddy.
Only general advice I have is remove all knots from the arms and try to anchor them somewhere high like between the shoulder blades (adding an extra rope is worth it). And get used to tying the arms tighter, learn where your rope buddys pressure points are and avoid them. It isn't always obvious where they are, I for one actually prefer the cinching of the arms go really high (almost into the armpits) and really low, the middle of my upper arms cause me to loose feeling because the muscle there pushes my veins inwards.
I have a few pics on my FetLife (same name as I use here) with both types in use. Don't know if it'll help you but by all means take a gander.
Good luck with your rope buddy
!
!