....... , my pen friend, ......... announcing new solidary campaigns for supporting the poor is gifted.
There is something called Reduced Adjective Clause where the relative pronoun can be omitted along with the verb to be and replaced by the present participle (-ing)
in this case we cannot replace Martin with him or her so whom
is unacceptable in this sentence however there are cases where we can omit a relative pronoun 'who is' only if it doesn't change the meaning of the sentence
so if we say Martin, my pen friend, who is announcing new solidary campaigns for supporting the poor is gifted. it can be acceptable ps
. why using who? Martin is subject and can be replaced with he
so here relative pronoun is omitted and instead of it there was added NOW
which doesn't change the meaning since now is used for present continuous to indicate action happening at the moment or it was in the middle of the action.
so the sentence can be like that Martin, my pen friend, now announcing new solidary campaigns for supporting the poor is gifted.
Here is another example on omitting the relative pronoun
.The boy who is
playing football invited me.--> The boy playing football looked at me
as you see two sentences have the same meaning