The friendship must have come in the weeks before Grave Peril. It is mentioned somewhere in one of the first chapters, that they have been hunting ghostly appearances for weeks together.
The reason I originally asked the question is because, in Grave Peril (at least the audiobook), it says they had met 5 years earlier. 5 years seems like an awfully long time. (I wonder if it was a typo and was meant to read 5 months...) What was Harry even doing 3.5 years pre-Storm Front (I think Grave Peril is about 1.5 years after Storm Front)? According to the timeline, Harry was 26 during Storm Front, would have turned 27 around Fool Moon and then 28 around Grave Peril, so, according to Grave Peril he was 22 or 23 when he met Michael.
According to the timeline:
6 BSF: After leaving Ebenezar at age 19, Harry roamed around for a while (between nine and twenty months) before winding up in Chicago.
5 BSF: At age 20, Harry arrives in Chicago. Before he goes to work at Ragged Angel, he takes a number of jobs, one of which was as a dance partner with a senior-citizen organization.
4-5 BSF: At age 20-21, Harry starts working with Nicholas Christian (“Nick”) at Ragged Angel. He spends three years there.
4 BSF: Susan becomes a reporter at the Arcane [5 years before Grave Peril, DM 198]
— The last time Charity sees Harry before the events of Grave Peril. Michael is injured, and Charity thoroughly chews Harry out.
3 BSF: Harry finds Mister the Tailless Kitten in a trash can. [SF 88] (Thanks, Amber!)
2-3 BSF: A Restoration of Faith. Harry meets Murphy. He has 6 months left of working at Ragged Angel. Faith Astor is 10. At this time, the Vargassi family and the Jamaicans rule the Chicago mob scene.
2 BSF: At age ~23-24, Harry opens his detective business. Ortega indicates that this takes place 5 years before the events of Death Masks, which takes place in 3 ASF. [DM 13]
2 BSF: Shortly after that, Harry meets Susan (it’s described as a “couple years” before the events of Grave Peril). From SF: “She had been tracking me ever since interviewing me for a feature story, right after I’d opened up my business.”
So, yeah, according to all of this, Harry met Michael several years before Storm Front and then Michael just wasn't in the loop in Storm Front or Fool Moon. Could easily be explained by Harry being a little scared of Charity's wrath. I'm still interested in how he and Michael met, since it was pre-Restoration of Faith, and Harry must have been really inexperienced at the time. Too inexperienced to handle the kind of thing Michael typically fights.