in case you do not have a written contract about the insurance it will be quite difficult to get any reimbursement as before court you will need to prove every detail the salesperson told you including the fact that the person understood that the insurance was an important issue for you.
It will all depend on your friend and it is quite likely that any judge will be very cautious in believing everything your friend states, because otherwise this way anyone could enforce a contract before court just by stating its content.
However, in contrast to most Common Law countries in Germany there is no "parole evidence rule" and it is theoretically possible to prove oral supplements of a written contract.
You should see the existing written contract. Check, what your friend remembers and then show up with your friend at the store and tell the manager the story. You should also make a written complaint and ask for the insurance benefit until a certain date (f.e. 4 weeks in advance).
If it doesn't help, you'll need to contact a colleague. Bear in mind, that chances are not too high to achieve something.
If you do not earn a lot of money and live in Germany, you may have most of the lawyer costs covered by so-called Beratungshilfe (you'll first need to ask for it at the local Amtsgericht).
You'll need to carry with you proof of your earnings and running costs (for example rent) and proof that you tried to solve the issue by yourself (for example the letter to Gravis).
Raub includes violence...
Theft (Diebstahl) does not.
It will be essential whether it was clear for the vendor that you desired insurance for theft.
You should try to make your point clear at the management in written by explicitly putting a deadline for the payment of compnesation or replacement of the phone.
Don't forget to mention you friend as witness.
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