Yes, Lief is eligible for Flexible Spending Account (FSA) reimbursement. The Internal Revenue Service (IRS) determines what are considered eligible expenses for all Flexible Spending Accounts. A listing of eligible expenses can be found on the FSAFEDS website.
According to this website, yes, medical monitoring devices like the Lief Smart Patch are eligible as an FSA expense:
When you purchase a Lief, you receive an itemized receipt in your order confirmation email, which can be submitted for FSA reimbursement.