I don't want to buy a Windows 10 key if I can use my friend's key
What you are suggesting is illegal. A OEM Windows operating system can only be used on the computer which the manufacturer installed it. If can not be transferred to another computer.
Please read the following Forum rule.
No subject matter will be allowed whose purpose is to defeat existing copyright or security measures.
But the thing is that there are no preventive measures according to this
"Thank you for using Microsoft Answers forum. Your product key will work for up to three separate computers, so if you want to install the the RC on another PC, you can re-use the same key."
However, I did find this (Below) and I see your point. I'll gladly purchase a key of my own in the future for this PC. For now the free windows 10 installation with the watermarks will suffice until I can afford a new key.
"I realize that in the past, you could get away with installing one copy on multiple machines because MS never had anything in place to check if the same key was used on multiple computers. Doesn't mean it was right or legal... it just means you got away with it. "