in a minor car accident, up to what extent is my brother liable?

last night, my brother was driving home and accidentally dented another car. he went down and spoke to the lady driver and asked her if she wanted to settle it by insurance or amicably. my brother accepted his fault and said he will pay for whatever damage there was though there was only a minor scratch/dent. nevertheless he called his fried to lend him 5k and gave it to the lady driver. my brother asked if she was ok already with that and she acknowledged and accepted the money in front of my brother's friend.
early this morning there were 2-3 calls recorded in his answering machine from the husband. my brother was not able to answer the calls.
i would like to know if he is still liable to the said party? my brothers fault is that he did not let the lady driver sign and receive the 5k? i this the husband is calling to harass my brother.
could you let me know please what we could do?
thanks you

the problem is that he did not have her sign a release. i would agree that 5k is a very reasonable amount for dent, but that also depends where it is fixed.