There's no need to involve a solicitor. Simply type out the following:
"This is the first codicil to the will dated [date of original will] of me [full name of your friend's mother] of [her address].
1. I revoke my appointment of [name of solicitor] as executor of my will.
2. I appoint [your friend's full name, or that of another chosen executor] of [their address] as executor of my will.
3. In all other respects I confirm my will.
Date:
Signature:
Signed by [full name of your friend's mother] in our presence and then by us in hers:
[Signature of first witness] [Signature of second witness]
[Printed name of first witness] [Printed name of second witness]
[Address of first witness] [Address of second witness]
[Occupation of first witness] [Occupation of second witness]"
Notes:
(i) your friend CAN be executor of his mother's will. (It's common practice. I was the executor and sole beneficiary of myfather's will) ;
(ii) you friend must NOT act as a witness (otherwise he can't inherit) ;
(iii) the witnesses MUST actually see your friend's mother sign the codicil. Their signatures won't be valid if they don't.
(iv) the inclusion of the witnesses occupations (and, indeed, of their addresses) is optional; it's just common practice to include them.