No endorsement required to work on a turbine engine (or turboprop/turboshaft engine) from a maintenance point of view. There's a been a lot of talk about type ratings for mechanics. I'm not sure where that effort is headed right now, but it's always rumored.
There's no additional logbook endorsement for a pilot for a 400 hp engine, vs. a 200 hp engine.
I do operate a turboprop single (1,200 shp, and 1,500 shp). The government requires aircraft-specific carding to fly it, which is a certification rather than an endorsement, but there was never any endosement required.
14 CFR 61.101(g) requires that a recreational pilot with less than 400 hours get a logbook endorsement from an authorized instructor if he or she hasn't logged any PIC time in the prior 180 days.
61.195(h)(3)(ii)(a) is part of several provisions, any one of which a flight instructor must meet to participate as an instructor teaching a flight instructor course, in order to teach (I know that could have been worded better, TonyC). One of the criteria an instructor can use is 400 hours of instruction given.
Those are the only two places in Part 61 that 400 is even mentioned. 61.31 makes no mention of the requirements your mechanic friend suggested. The only requirement related to that would be the standard high performance endorsement found in 61.31(f).
