Have you emailed her yet? Just curious, it's not clear to me if you're waiting on a reply or still deciding. I have no prior experience, but I have a hunch that some therapists would and others wouldn't friend ex-clients.

On a side note my therapist is always mentioning friends, it sounds like he is just loaded up with buddies and I've always wondered where these people come from. In contrast it is very hard for me to make friends... I have some 200 FB "friends" and maybe 3 real life ones.