I don't suppose your friend's footage is all 3840x2160 or 2160x3840 and you're willing to edit at 1080x1920? If so, you can crop a horizontal frame (landscape) to vertical (portrait) without up-converting.
Or,
Drag and drop one of the portrait clips onto the New Item icon at the bottom right of the Project tab. Rename the resulting Sequence such that it has a better name (it will have the same name as the clip that you dragged and dropped). Then assemble your edit as you would normally. When you get to landscape shots, you'll have to go into the Effects Controls panel and increase the Scale such that the landscape picture fills the portrait frame from top to bottom. To avoid scaling up the video, you could try to do a vertical split screen for those segments of your edit. If you're seeing the landscape shots sideways, you will need to adjust the rotation as well. (If you're seeing the portrait shots sideways, you'll have to manually adjust the Sequence settings such that the Sequence is portrait.)
The picture quality of the scaled shots is going to look soft. Red Giant Instant 4K (there's a trial version available for download at their web site) will do a much better job with the up-scale than the Premiere Pro Scale parameter. You could also use After Effects' Detail-preserving Upscale effect for better results, although - as much as I love AE - I think Instant 4K looks better.
-Warren