yes do be careful seems she is a bit touchy about it, although entitled to her opinion thats all it should be.
I have thought this a few times and have sat opposite my new fella and thought would i tell him but at the moment no i would not and glad so far as when i have got to know the fellas better the relationships did not last beyond six weeks..

We dont ever have a moral obligation to tell anyone anything.  We may well have a feeling we should tell someone we are having sex with that we have other partners so he knows to get regular checks himself.  That is personal to each person though so some might and some wont.
I would always tell someone, OK I owuld not tell him I was an escort as it is far too early to be doing that but I would tell him that as we are just sex partners at the moment I will be having sex with other people, this way i can also gauge how he will react later if it turns into a relationship and I have to tell him I am an escort.
I do it because I know I would want a guy to tell me if he was having multiple partners whilst having sex with me so I had the right to choose to continue sex with him or not.
It then gives him a choice if he wants to have sex with me or not rather than me taking his right to choose away from him.
Like I said that is just what I would do but everyone is different and we have to do what feels right to us at that time and at the end of the day only you know what feels right to you.

I can see it from your friends perspective as she probably feels you are taking away his right to choose as he does not have the correct knowledge to make an informed choice
I can also see why you would not want to tell him you have sex with other people as he may decide to walk away