Question about SSRI use and psilocybin

Hi. A relative of mine has major depression. He has had it for a couple of months now, and had been on meds (SSRIs) for 8+ months. I had offered him to try psilocybin, because it has helped me and other people I know with depression. Thing is, if I am not mistaken, SSRIs inhibit serotonin reuptake. This means that there is more serotonin in the synapse, as the neuron cells are not reuptaking the serotonin. Psilocybin on the other hand, is a serotonin agonist, if I am not mistaken. By no means am I an expert on neuropharmacology, but I am studying neurology. My question is: Is the use of SSRIs and psilocybin together safe? Does it depend on the doses? Do you guys have any research on this topic that I might find helpful? Should the SSRI use be discontinued before trying psilocybin? Things like these.. any replies would be very much appreciated. Thanks