Is it safe for a person with a lower back injury (two spinal fusions) to sea kayak?

Sure: Use pain as your guide low impact great core workout down side lots of sitting sometimes rough depending on surf.Start with short trips on smoothe water.

Yes if fused & rehab: Yes as long as they have no significant pain and are cleared by their surgeon in terms of the healing of the fusion and have gone through a structured rehabilitation program. This should be tested with short distances first with the appropriate safety measures ie life vest &/or partner. Exercise is always beneficial as long as proper mechanics and posture can be maintained.