How to escape file path in SED?

sed -i 's|'$fileWithPath'|HAHA|g' file

Single quotes define a string literal. Putting the variable outside the literal allows the shell to expand that part.

Also: if you are going to parse paths, use a delimiter in the sed command that doesn't confuse with the directory delimiter "/".

A better way for literal strings with forward slashes:

sed -i "s|my/path|my/other/path|g" myFileOfPaths.txt

In circumstances where the replacement string or pattern string contain slashes, you can make use of the fact that GNU sed allows an alternative delimiter for the substitute command. Common choices for the delimiter are the pipe character | or the hash # - the best choice of delimiting character will often depend on the type of file being processed. In your case you can try

sed -i 's#'$fileWithPath'#HAHA#g' $file

The character 'g' after last # is used to change all occurrences in file if you need to change only first occurrence then remove the 'g'.