The OP's question is not whether it is ok for a friend to recommend a T to their friend. The OP's question is about a scenario where someone who IS a T has a friend in need of therapy, and they refer that friend to someone who works in the same practice as them (the friend-T), AND with whom the T-friend is friendly socially. Then after the client has started therapy, the T-friend begins a romantic relationship with their friend's T.

Therapists, at least qualified psychologists in the UK which is what I know about, have to adhere to a code of ethics. They would be taught exactly why, in detail, during their doctoral course. there is no way they could qualify without knowing about this. These kinds of dual relationships are in fact a breach of professional ethics, and for good reason. If you look online you can see published notes from hearings of the Healthcare Professional Council and the Royal College of Psychologists which will illustrate firstly that this is clearly against the rules of the profession, secondly unfortunately it will show a wide range of the kinds of things that can go wrong in therapy when professional boundaries are breached and the kind of harm that can result.

Great that it turned out really well for you being recommended by a friend. I hope you can find another good T.

I really hope the situation the OP wrote about doesn't happen to someone with serious attachment difficulties, trauma history, a lot of transference and the like. The risk to them would be like playing with fire. It's understandable that they might not realise it before they begin therapy, but the T really has no excuse for taking such risks, in my opinion.