Nothing else is guaranteed in NFL. Getting to 2 round or higher makes a huge difference.
One year older? Meh. Most careers are over in a few years before guys hit 30 anyway.
And if you do make it to 30, then you have done something right and are rewarded handsomely for it.
If his intel says one more year can help with the big jump in guaranteed signing bonus, he should come back (and get an insurance policy too).