Yeah. Your friend is jealous! Seriously, a TON of people (me included) use these for Mash tuns, and apparently have been for years.
The plastic is HDPE (food grade).
It retains heat wonderfully without external sources.
Oh yeah, did I mention that it was cheap? With a valve, you are probably looking between $50-75.
As for plastic leaching, remember that the mash is only in there for an hour or so. Even BetterBottles are PET plastic and fine for fermenting in.
Does your friend ever drink bottled water or soda from a plastic bottle