My buddy has an older Hoyt bow and with the limb bolts tightened all the way down he's still not getting the draw weight he should be. Could this be due to the string being stretched over time? Would twisting the string a few times to shorten it increase draw weight?

It's probably the cable rather than the string. I'd replace both.

You're completely backwards. If only the string creeps (not stretch) the drawlength gets longer and the weight goes up. Probably what's happened is that both the string and cable have crept over time decreasing the prestress on the limbs, thus the weight goes down, even though the drawlength may have stayed essentially the same.

Just how much is the weight down? Most bows go a couple pounds over the rated weight, but most companies note that weights can vary as much as 3# on the downside, too. In reality, ask yourself. Do you think a deer knows the difference between 70# and 67#, or 60# and 57#. A passthrough is a passthrough.

The bow is maxed out in the low 60's which would seem significant to me. I have no doubt that at that weight it will kill deer, in fact it has killed several, but I also think that an extra 5 pounds or so of draw weight would make for a flatter shooting arrow and less yardage estimation errors.