It is a violation of the Fair Housing Act for any person to refuse to make a reasonable accommodation in rules, policies, practices, or services, when such accommodations may be necessary to afford a handicapped person equal opportunity to use and enjoy a dwelling unit, including public and common use areas.
Individuals who are disabled may request a reasonable accommodation in housing. A person is considered disabled if they have a physical or mental impairment which substantially limits one or more of their major life activities, if they have a record of having such an impairment, or if they are regarded as having such an impairment.
Severe asthma counts as a disability. It's covered by The Equality Act 2010, which protects people from being discriminated against.
I would replace the flooring and not charge them. Not worth possible legal expenses.