A good question.  I had never heard of this particular reference from Philo.  I can think of three possibilities:

1. For some odd reason, Philo thinks of Isaiah as a “kinsman and friend of Moses.”  This seems a bit unlikely from our perspective, but perhaps this would have seemed a fairly normal thing to Jews in the first century.  Surely, Philo was aware of Isaiah 1:19, and perhaps his statement is a reference to Isaiah.

2. There was a certain unknown (Miriam? Aaron?) prophet in the time of Moses who said this, but it was not written down, but, somehow, Philo knew about it, and, therefore, Isaiah, as well as Philo was copying that prophecy.

3. Philo is simply wrong.  He is making a statement which is not backed by any good evidence.  There was no kinsman and friend of Moses who said this.

Of the three proposal, it seems to me (but, of course, you can judge for yourself), that the least likely is the second option.  It requires too much speculation about something we have no evidence for whatsoever.   Between the other two options, #1 #3, I really cannot decide for myself.  Perhaps an expert in Philo could help decide that, but I am not an expert on first-century Jewish author Philo.

So, my answer to your question is that it seems to me that it is quite unlikely that Isaiah is quoting a previous prophet, but I cannot absolutely rule it out.  By the way, it is not unusual that one prophet quotes another–either that or God gives the same statement to both.  There are many examples of this.

John Oakes