There is an important difference between one who is mourning for a relative other than a parent and one who is mourning for a parent regarding shaving. Regarding mourning for non-parents, one may shave immediately following sheloshim. However, concerning one mourning for a parent, one may only shave after reaching the point of “יגערו בו חבריו” – either when someone reprimands them for their unkempt appearance or when their appearance is considered socially unacceptable (SA YD 390:4).

Once the sheloshim period concludes, either after a full thirty days or due to the occurrence of Yom Tov within that period, one only needs to let their beard grow until “יגערו בו חבריו,” namely until someone reprimands them for their unkempt appearance.

Although not stated in SA YD 390:4, the general assumption in the poskim (based on a number of Rishonim – see Beit Yosef YD 390, Rabbi Akiva Eiger 390, Igrot Moshe Yoreh Deah 3:157) is that גערה does not work before 30 calendar days are over (and regarding the Rema’s position that גערה is three months, see Pichei Teshuvah 390:4, who cites Noda Bi’Yehuda that this only applies for haircuts. Beards can be shaved sooner).

So, according to the general psak that 30 calendar days are needed, we would need to base this allowance on makom hefsed (financial loss), which it definitely is (see Pitchei Teshuva YD 390:4, Kol Bo 2:122, MiPeninei HaRav p. 207). I would advise also doing the גערה which would work according to a number of Rishonim. This could be accomplished by having someone say to him – “Jon, you are looking pretty scruffy. It’s not appropriate to be going around with that beard. You could really use a shave.”