Yes, there could be a danger, and here's why.
Before practicing Reiki, a person must be attuned, which is a ritual during which the teacher activates the "universal energy" within the student. According to the Usui and Seichem Reiki Training Manual, “the Reiki attunement is a sacred spiritual initiation that connects the initiate with higher levels of consciousness and permanently connects the initiate to the Reiki source” (which is unnamed), and that “the attunement is also attended by “Reiki Guides” and other “Spiritual Beings” who help implement the process.”
By process of elimination, we know that these "Spiritual Beings" cannot be from God because He specifically condemns occult practices in Scripture, and we know they are not disembodied human souls because they are judged and sent to their final destination at the moment of death. The only "beings" left are demons. (You can read more about this here.)
This is why persons who receive Reiki treatments are exposing themselves to occult forces depending on the practitioner’s background, their level of initiation, use of mantras and symbols and whatever spirits they may be channeling.
However, we also know from the teachings of exorcists such as Fr. Gabriele Amorth of Rome that evil spirits can inhabit houses/buildings by means of the people they are influencing. In fact, Fr. Amorth's practice includes the use of prayers from the official Ritual which ask the Lord to protect places from evil influences. He also walks from room to room, exorcising each one by reading the first part of the exorcism for individuals after modifying it to address a building. Father also exorcises places where occult activities have taken place (An Exorcist Tells His Story, pg. 125).
So, yes, as long as there are people in the building who rely on "Spirit Guides" to help them channel energy during Reiki treatments they are summoning demons into that building who may or may not choose to harass anyone else who might be present.
If I were in your shoes, knowing what I know, I would stay out of that building and find another massage therapist.