Yes, I realise that they are not in a dual relationship right off the bat when the T and the friend-T start dating, although it would be almost a dual-relationship-by-proxy I would say - how many days is it likely to be before the T heard the friend talking about the client as a friend, calling them, including them in an invitation etc. They would be in a dual relationship from the first time that e.g. they were at the same party. The dual relationship thing seems unavoidable in this scenario IMO.

Of course it *may* be fine but it *may* also cause a very great deal of harm to the client. What if they had been actively suicidal, andvthis T was the first person with whom they had started to stabilise. Or this T is the only person they allow themself to become close to and trust a little, perhaps they disclose some kind of abuse for the first time with this T. The potential for harm when T is included into this person's social circle is huge. Also we may be talking about a sociable person with many friends and acquaintances, or we may be talking about this friend-T being the person's only friend. If things go wrong with the therapy, I doubt the friendship would survive.
These are unknown factors and would probably be unknown to the T on the day of the first session, but that's the point of ethical guidelines. It's up to the T to know how potentially vulnerable a client may be, in terms of the potential vulnerability of their mental health and their life in general, it's up to the T to understand about transference, attachment, and other strong feelings that can come up during therapy, it's ultimately up to the T to know about how important boundaries are in therapy and to be clear from the outset and not take these kinds of risks. The friend-T in this scenario should have known all this as well.