What should I provide for proof of accommodation while staying at a friend's place in Cambodia?

Write a cover letter saying that you are staying at your friend place. If your friend got a rental contract then it'd be great if you can include that in the documentation. easy peasy

There will be two things.

1. You send the picture of your friend home and address and explain that you are staying with your firend. (Your friend house rental contract may be needed if your friend rent that house)

2. You book a paid booking for 2-3 days and send the hotel name and hotel address. (There may be some risk that the officer will check with the hotel for your stay). In case they ask you why only 2-3 days, you can tell them that you don't want to stay in one place for too long and you are visiting other places)

Other that that, I cannot think any other options.

Or hotel that in you mentioned for stay in Thailand?.

They meaning about hotel in Cambodia that you stayed and waiting for your visa right?