Tricky. 125# isn't that heavy, so there won't be a huge amount of compression on the bow arm once he gets used to the weight. There's definitely no point in playing for risks, so I'd say you'll have to make a bow around 84" long.
I can tell you for a fact that the longest bow found on the Mary Rose was 2113mm, or 83.2" so definitely nothing around 84" long. It depends I guess on whether his priority is having a perfectly accurate Mary Rose replica, in which case his draw length will suffer, he won't get his full potential power and may well risk over-drawing, or whether he wants a powerful, peak-performance warbow which will let you make it 84"