No 1e module will have enough experience opportunities to bring a 5th level character to 10th by the end. The range is a spread based on the number of PCs.
My groups have never had that specific issue but I would either suggest the direct approach and have the players let him know your concerns or the next DM can use the old "You have earned the wrath of the DM" approach and have your camp, keep, etc. raided and the excessive money and treasures...
I've never been a fan of crits. That was one of the draws D+D had for me. MERP and other systems with crits had too many characters die on fluky rolling. They were fun to read if it was someone else though.
You could go old school with B2-Keep on the Borderlands and have a change in leadership either in the beginning or when the PCs head back to the keep to resupply.