Below is a link I've kept in my 'remodel folder'.
After reading it you may decide to paint the frame!

Here is a link that might be useful: staining pine

"Or should I just have him use a harder wood to build the frame?"

Yep... that is the option I'd choose!

I've stained many types of wood and I would recommend going with a harder wood. Pine doesn't absorb stain evenly and it is usually because the wood is too soft and too new. If you do go with pine, at least first use a wood conditioner on the frame, then stain. That won't guarantee success, but it helps alot.

Ugh. That's what I was afraid of. Thanks ladies, I'm sure you've saved me a lot of time and frustration :)

I still think you could be okay. You are staining a frame -which is relatively narrow and I do not think you would see unevenness (if that is a word) as much as you would on a large piece. Since he is a carpenter and you already have the wood, give it a whirl. He can probably make the frame in minutes.
Do use MinWax wood conditioner as Paster said. It is a great product. Pine also tends to darken after a while even after stain and poly.

All of our doors and trim are pine. Stained with SW Walnut Wainscot. It is very dark and the wood stained evenly - one coat.

I have some pine pieces in my kitchen that are stained a medium color but I don't know the name of the stain. It is also evenly.

Here's another option...

After sanding, seal the unfinished wood with polyurethane.

Then apply Minwax Polyshades to the wood. This is a colored varnish that will not penetrate the wood. You will get the color you want without the stain absorbing into the wood.

Do this before you assemble the frame, and make sure the frame is lying flat when you do it.

I took woods in grade school and high school and learned just a wax steel wool the pine doesn't soak up the wax still have that pine color other than that pine is just for painting

There is often confusion as to what the best stain for Pine might be. Some say that it's impossible, but all woods have their own set of different stains and finishes available!

I would recommend a penetrating stain for pine. Best avoided are any water-based stains because they will become permeable to the wood's pores, leading to wet spots on your floor before long. But if done right and with an oil or latex-based dye (or lacquer), these products work well too!