If the testator has given the jewellery as a gift to the middle daughter (rather than retaining ownership of it and just letting her look after it) the part of his will which gifts it to the oldest daughter is invalidated and she gets nothing. She can't be given anything else instead although, of course, your friend or anyone else who benefits under the will is free to give some of their own inheritance to whomever they choose (including the oldest daughter).
He needs to either write a new will or, at the very least, a codicil to the existing one. He doesn't need to use a solicitor. Something like the following will do:
"This is the first codicil to the will dated [date of original will] of me [full name of testator] of [full address of testator]
1. I revoke the bequest of my [description of jewellery] to my daughter [full name of oldest daughter].
2. I give [relevant sum of money] to my daughter [full name of oldest daughter].
3. {NB: This part only needs to be inserted if the jewellery hasn't already been given outright to the middle daughter} I bequeath my (description of jewellery) to my daughter (full name of middle daughter).
4 {or 3, as appropriate} In all other respects I confirm my will.
Date: [Date that codicil is signed]
Signature: [Signature of testator]
Signed by [full name of testator] in our presence and then by us in his:
[Signature and printed name and address of first witness, who must not be a beneficiary under the will]
[Signature and printed name and address of second witness, who must not be a beneficiary under the will]"