What is the rule for pocket depth?
At the class I took at Heartwood school where they taught us beam sizing and other engineering stuff, they gave us this rule of thumb for floor joist to sill beam situation:
We also learned that you should size the joist first and then size the timbers to hold up the joist next. And work you way down to the foundation, making timbers large enough to hold everything up properly.
If that is the case, then maybe the sills in this extended frame will have to be deeper to be stronger.
As I mentioned just adding a supporting sleeper under the middle may solve the problem without changing a lot of joints and beam sizes.
He would just have to make it so his foundation would support the ends and add a pier to the middle.