No software would take the private key in (tried a dozen), and trying a WiF decoder showed it's invalid (even though it "looks" right, starts with 5K, right length, etc).
Let me guess, it's:
5Km2kuu7vtFDPpxywn4u3NLpbr5jKpTB3jsuDU2KYEqetwr388P, right?
I'm sorry to tell you that it's
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141e4770308 which is out of range.
Sadly, no one can recover those 3.7 BTC that your "
friend" accumulated:
1FYMZEHnszCHKTBdFZ2DLrUuk3dGwYKQxh