We had a similar situation when we had stumps drilled out of our yard, 25 of them. We were told that the mushrooms were part of the wood chip decomposition process and that it would take 3-5 yrs for the rotting wood process to run it's course. We very carefully took out the mushrooms we could, but they kept coming. I hate to say it, but based on the advice we were given, all I can recommend is patience. Roundup will kill whatever grass your friend has growing in that area.