Allergies can be considered a protected medical condition, so Susan shouldn’t be excluded from the pool of candidates because of her allergic reaction to Fuzzball the cat.
Prior to hiring, you can discuss reasonable accommodations with Susan. Is it a reasonable accommodation to remove Fuzzball the company cat from your office environment? Legally, it probably would be considered reasonable: Fuzzball isn’t essential to your company’s work and there would be no undue expense involved. From an employee morale perspective, this decision is more difficult.
Would it be a reasonable accommodation to let Susan work at another location? On the occasions that she must come into the lobby, she can wear an allergy mask and the owner will keep Fuzzball at home that day.