Can a friend become a therapist? According to the article Can Clients and Therapists Be Friends?, a therapist shouldn't be a friend because dual relationship will interfere the efficiency of the therapy, and the imbalance power formed in therapy will never fully disappear. However, would it be true in the opposite direction? That a friend does therapy to a friend? This time, the friend relationship comes first, so there will be no worry about the imbalance power. 

I think there are two scenarios:


The therapist-friend is just a friend with more knowledge, and the therapy is just a talk with more advice from one side
The therapist-friend cannot become an authority that is necessary in certain cases (personality disorders, for example) 


The person can also be a family member, a partner, a colleague, or even an acquaintance. The point is that the relationship is not formed in a professional therapeutic setting.
The article you linked to (Holmes, 2018) is talking about both senarios.  The therapist must not be a friend before or during therapy sessions.  Dual relationships can interfere with the efficiency of the therapy due to the fact that it can create transference and counter-transference issues.

Transference

Transference as a psychological concept refers to the redirection of feelings about one individual to another individual (Short &amp; Thomas, 2015).
Another definition would be


  Transfer of feelings originally experienced in an early relationship to other important people in a person’s present environment (Luborsky et al., 2008)


Transference is feelings about another person (including the therapist) being transferred (maybe acted out) towards the therapist.

Imagine a therapist, who is also a friend, says something which upsets the client unintentionally because 


  (s)he shouldn't say that. (S)he is my friend!


This is where the friendship can affect the client/therapist relationship.

Counter-transference


  The term ‘counter-transference’ has been applied to a large number of psychological concepts in the therapeutic setting (Little, 1951)
  
  
  Counter-transference refers to a specific unconscious attitude towards the client in response to transference from the client (objective transference – responding to
  transference from client)
  Counter-transference refers to repressed elements of previous events leading to the
  displacement of past feelings from the source to the client (subjective transference
  – similar to transference in the client but acting in the opposite direction) (Short &amp; Thomas, 2015).
  


Counter-transference is feelings about another person (including the client) being transferred (maybe acted out) towards the client.

Imagine something is said by the client, who is also a friend, which changes the therapist's view about that friend.  Maybe the revelation changes the dynamics within the friendship they have.  This can affect client/therapist relationship through both counter-transference (feelings about the revelation from the client being bounced back to the client) and transference (resulting feelings from the client being bounced back to the therapist).

Dual relationships, no matter how and when they form cause big problems within ethics and efficacy of the therapy sessions, and therefore they are not to occur.

A friend (or family member) should never give therapy to another friend or family member.  If dual-relationships do form during sessions, this must be discussed immediately (or as soon as reasonably practicable) with the clinical supervisor in order for discussion to take place regarding signposting and transfer to another therapist.

References

Little, M. (1951). Counter-transference and the patient’s response to it. In: Langs, R.J. (ed.) Classics in psychoanalytic technique, pp. 143-151.

Luborsky, E.B., O’Reilly-Landry, M. &amp; Arlow, J.A. (2008) Psychoanalysis. In: Corsini, R.J. &amp; Wedding, D. (eds) Current Psychotherapies. Belmont, CA: Brooks/Cole.

Short, F. &amp; Thomas, P. (2015). Core Approaches in Counselling and Psychotherapy. Hove: Routledge