The referral itself is a little suspect, although on the other hand a friend who knows therapists personally would know that a therapist would mesh well with me, so I understand recommending someone from your same clinic.
If I were the client in this situation I would expect both my friend and my T to be upfront and honest with me immediately. I would want to discuss with my T what this would mean for our therapy relationship. I wouldn't drop my T necessarily, but I would be extremely wary -- looking out for things my friend might know about me that they would only know about me because my T told them (or vice versa). One sniff of breach of confidentiality and I'm out.
If it made me uncomfortable, I would leave, but wish no ill will upon my friend or therapist (assuming no confidentiality was broken of course). Even if it's inconvenient for me, who am I to be angry that two people have found love?