Can't select treeview node from iframe

My problem in short: I don't know how to keep the selected node from a treeview in sync with the corresponding page in the IFrame.
So, I've got a Treeview, with 2 treenodes, and an Iframe, both in default.aspx. Further I've got 2 pages, page1.aspx and page2.aspx, which corresponds with treenode1 and treenode2. When I click on a treeview node, 2 things happen:
1. The corresponding page appears in the IFrame;
2. The selected node gets bold.
That works allright.

When page1.aspx is shown in the iframe, it has a link to page2.aspx. When I click that, the same 2 things need to happen:
1. Page2.aspx needs to appear in the IFrame;
2. The corresponding node needs to get bold.
The first thing works fine. But I can't figure out how to get de second thing to work...?

I do know how to change the selected treenode programmatically, but I don't know how to call that function, which is in default.aspx, from the page in the frame (page1.aspx).

In the attached file my project in short. In the code behind of default.aspx is a function to change the selected node.
Treeview-problem.zip