When I left a previous company in the UK, my pay was docked in exactly the same manner. I just accepted that was part of the contract and that was that. It was all very amicable - we just agreed how much I owed them (the lady who did it was very generous in her interpretation of the bond!) and that was that. The great advantage of doing it that way is that you do not pay tax - if your pre-tax salary is £8k then you just do not get paid for you last month and off you go. If you have to find £8k in cash then you have to earn that and pay a pile of tax - it seems your friend is actually being done a favour.
Another factor in this, alas, is that historically a number of pilots have left a company and then just refused to pay their bond - that involves lawyers and hassle that employers do not want. Therefore I have considerable sympathy with the company. If you take a job that involves being bonded and then leave early, I have difficulty seeing what the problem is if they take it off your pay at the end. The bottom line is - be careful what you sign up to.