I don't know about your friend, but as for the "reported" cases in your area, I imagine they are like the rest of the reported cases which are mentioned in my links in which upon further investigation there was no fentanyl in the cocaine but possibly some other substance. Hearsay from a bunch of cokeheads is less than reliable.
I have witnessed plenty of opioid ODs, and foaming at the mouth is not the first way I would describe it after witnessing it, I know it can happen a little if the person is gurgling or choking for air for some time but it's not really "foam" its just spit bubbles and drool and it really only happens when the person falls out in some weird position not if there are people around him who lay him straight; usually you are just out cold, breathing fairly softly if at all. I don't want to call bullshit if you witnessed it, perhaps your friend did OD on something even coke(I have seen people on 3 day coke binges straight drop out and wake up an hour later in the ER after getting fluids and meds) but as I said if this is just hearsay I wouldn't put much faith in it. I'm sorry, but a story from a bunch of coked up people about someone "foaming at the mouth" from "fentanyl in the coke" sounds like...well you know.
If you want to purify cocaine, fine go ahead, but purifying with the intention of removing fentanyl is a waste of time; and if you do it incorrectly it can make your cocaine more harmful rather than less but will probably just end up wasting your batch.
If it is happening, its happening very far down on the supply chain, get your coke from someone else. Changing your hook up is a far better and simpler solution than trying to remove fentanyl from every bag you get.