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How much was the stipend.
What services were performed
At that rate, the individual is not a volunteer, but is a consultant. However, they are probably being paid at rate lower than their standard billing rate. The amounts received for these services is taxable income to the recipient.
The main concern is the relationship the individual has with the entity they are providing services for. There are factors that should help to determine if the individual should be an employee or independent contractor.
I have provided a link to a Twenty Factor Checklist to Determine Independent Contractor vs. Employee Status
It is possible that a stipend could be tax free if the amount related to unreimbursed out of pocket expenses and had no correlation to services performed.
Excerpt from IRS web site, link below
Grants given to students, trainees, or researchers which require the performance of personal services as a necessary condition for disbursing the grant do not qualify as scholarship or fellowship grants. Instead, they are compensation for personal services considered to be wages. It does not matter what term is used to describe the grant (for example, stipend, scholarship, fellowship, etc.).