In the US, discrimination claims require a matching of protected class and action type, as summarized here. This comes about by Congress seeking to outlaw discrimination on a particular basis, and not always making the restrictions universal, for whatever reason (that is, the laws are politically primarily driven by the class, not the action). The two most likely potential applications would be pregnancy and familial status. As you noted, under FMLA there is no requirement to grant post-pregnancy leave in companies with small numbers of employees. So if you suspect that she was fired because she took maternity leave (and assuming the employee was eligible in working at least 1,250 hours for at least 12 months, and works at a place with at least 50 employees within 75 miles), then there is a possible case. There are also advance notice requirements, for the employee taking leave.
The relevant regulations are given here. 29 CFR 825.100 generally asserts an employee's legal entitlement to return to work after a specified leave. The regulations do not, however, say that a person returning from pregnancy cannot be fired, and what counts as a violation is not objectively spelled out in the regulations. Section 825.216 somewhat addresses the issue of an employees right to reinstatement. For example, under paragraph (a), the employee "has no greater right to reinstatement or to other benefits and conditions of employment than if the employee had been continuously employed during the FMLA leave period".
The procedure is to file a complaint and have a hearing. Since a substantial portion of the firm was laid off, unless it turns out that all of those employees were also pregnant, the company can easily demonstrate that the dismissal was not discrimination based on pregnancy, rather it was a neutral business decision. In addition, if your calculations are correct that this is not a covered business, then again this would not legally be discrimination.