I have no problem with it at all, as long as he watches and doesnt talk/interrupt too much! Thats why boundries, plan needs to be layed out prior!
I had a Japanese couple where the husband would watch and the minute after I was done he would hop on for seconds! His Japanese wife told me that he couldn't get it up without pills and it doesn't like to take them, the only way was to watch her with another man and then he would have her afterwards. It was a fun experience! Went on for a year until I moved to Miami.