Who should write appeal letter ?

[bluemoon2]

Hi , just a quick question here , my friend got Irish citizenship this year and living in Ireland , his non eu son, who is over 18 , applied for a visit visa to come to Ireland and see his father , my friend gave his son sponsor letter as well to support his application , but now he has got refusal and thinning to appeal the decision , so my question is who should write and lodge appeal letter ? my friend ( sponsor ) or his son (visa applicant ) ?
he is confused regarding this issue , any help is much appreciated.

[bluemoon2]

anyone there with any idea ?

[CR001]

son (visa applicant )

Because it is his application and he is an adult.