In the US, MBZ only offers their extended warranty to the original owner and
it must be purchased before the first 12 months or 12K miles.  The only
other MBZ option is the CPO Extended Warranty (formerly Starmark), but this
is only available when the vehicle is re-sold through an authorized dealer.
I suppose there may be some way a dealer could circumvent the rules doing
some paperwork to show where they bought the truck then sold it back as a
CPO vehicle, but they aren't supposed to do this and could get in "big
trouble" from MBZ if caught.  Otherwise, the warranty is going to be some
sort of 3rd party aftermarket agreement.

Many dealers have their own in-house warranties, and this may be the case
with your friend's offer.  I looked into the warranty that my dealer (Crown
of Birmingham) was offering for about $4K.  I would feel more comfortable
going that way over dealing with some anonymous national warranty company
that might go out of business or be a hassle to deal with.  At least with
the dealer's in-house warranty, you only have one party to deal with when
resolving problems.  In the end, I chose to "roll the dice" and not buy the
warranty, mainly because it was fairly restrictive in what it covered.  I
only plan on driving it another year or two at the most, so if it starts
giving me more than $4K in trouble, I'll just go ahead and trade.