The Halocho clearly states that someone who medically requires to consume chicken, at any rate if feasible, should endeavor to be Machmir & abstain from Ov 7th onwards, that being the day that they entered & pilfered the Beis haMikdosh; i.e. Until Ov 6th there are more grounds for leniency in case of necessity & medical issues etc.
BTW it is more important to fast on Tisha b’Ov and if this person who is allergic to Dairy & Pasta will be so weakened by skimp eating for 8 days prior, it is more important to eat enough to ensure his/her ability to have strength to fast on Tisha b’Ov.