It is extremely common for parents to think the Frontier 85 is on the top slot when, in fact, it isn't. I am not sure why that is, but I would definitely have her check again. Like Featherhead said, make sure she's loosening the harness and then adjusting the height.
If this is just a crazy giant of a kid, and if he really does outgrow the harness before he's ready to be boostered, I would probably consider an EZ-on 86Y harness along with the Frontier in booster mode with the SecureGuard attachment. She might want to call Britax to confirm that they would be OK with this set-up, but I don't see any reason why it wouldn't work nicely. EZ-On approves the use of their harness with a booster as long as the booster manufacturer OKs it. I think only Diono has come out and officially approved it... I might make the parental decision to try it anyway whether Britax is willing to stand behind it or not.