Getting rid of Bing

[#1]

I have a friend who has Bing on his laptop but would prefer Google. He tells me that he has tried to change to Google several times but Bing reinstalls itself. 

Advice please. 

[#2]

which operating system

[#3]

Which web browser?  If Internet Explorer, how does he respond to the pop-up asking if he wants to revert the settings to their defaults?

[#4]

windows 10, he has tried loading Firefox but says he gets a warning that the search engine can't be changed?

[#5]

Google is the default search engine on Firefox.  If he resets it to defaults I would have thought he would be okay. https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

[#6]

are you sure we're not talking about windows 10 in-built search (via cortana)?
that uses Bing and you can't change it either.

this has nothing to do with what you browsers uses though.