... I would usually suggest the fee was not the recommended one but "
Transaktionsgebühr: -0.00051881 BTC" this fee seems enough for it to not get stuck for 3 days.... strange.
...But 50K satoshi fee is high and it will just took a minute to made your transaction complete.
Until you realise that the transaction size was 2553 bytes... making his fee ~20 sat/byte... in the middle of a spam attack... where the recommended fee has been well over 150 sat/byte for the last 3-4 days... 
in any case, it looks like the transaction has dropped out of the mempool... so his friend should be able to resend it... with a proper fee!!?!
